bugfix: view: Fix Zulip crash during search.

[Rohitth007]

This commit fixes a bug which causes Zulip to crash when searching
Streams or Topics. This happens because when a search result is
empty, log is empty and because an empty ListWalker returns None,
it leads to a Type error when setting focus.

Commit 1: Fixes update errors and unread count edge case.

• This is caused when ZT recieves an update event while searching and
  set_count assumes that all widgets in the log are Buttons since
  the log is updated while searching.
• This also means that message updates are only sent to a subset of
  streams, topics or users. Which also leads to missed updates and
  negative counts.
• Since, we only need to update the count variable in the button
  object this is solved by using btn_list instead of log.

Solves part of #642

Commit 2: Fix Zulip crash during search.

• This commit fixes a bug which causes Zulip to crash when searching
  Streams or Topics. This happens because when a search result is
  empty, log is empty and because an empty ListWalker returns None,
  it leads to a Type error when setting focus.
• This is fixed by making sure the user does not enter the empty list
  when the search result is empty. The boolean empty_search is used
  to achieve this.
• This commit also improves the UI/UX when search result is empty by
  displaying Text Not Found in place of the empty list.
  The user can then correct their search and Not Found disappears.
• A search_error attribute is introducted in the pallete which
  colors Not Found in light red. The cross_mark symbol -
  STREAM_MARKER_INVALID is also added for effect.

Fixes #923

Commit 3: Name change for cross symbol for consistency.

• STREAM_MARKER_INVALID has been changed to INVALID_MARKER so
  that the cross symbol can be used in multiple places not necessarily
  related to streams. Eg: Search error indicator.

Commit 4: Change name of topics_to_display for consistency.

• The name of the variable topics_to_display has been changed to
  topic_display to maintain consistency with stream_display
  and user_display.
• This can be dropped it it feels unnecessary.

Commit 5: Test split for better readability.

• The tests for PanelSearchBox introduced when fixing bug crash while using search #923 is
  split into two tests for better readability.
• Both cases test_keypress_ENTER_focus_set and
  test_keypress_ENTER_focus_not_set have to be checked carefully to
  avoid the bug.

Commit 6: Erase caption when reattempting search.

• set_content('') is used so that when a user presses q for a
  second search the caption becomes empty.
• This makes for a cleaner UI while searching by removing caption
  Search Results when typing.

[Rohitth007]

@zulipbot add "PR needs review"

[Rohitth007]

@neiljp Does this need any more changes?

[neiljp]

@Rohitth007 This has great potential 👍 There are quite a few moving parts here, so I'd like to see them split into independent parts where they are independent, to make it clearer what each part does. The search reset seems like a separate matter entirely to the changes necessary for the UI change which resolves the bug?

[Rohitth007]

@zulipbot remove "PR awaiting update"
@zulipbot add "PR needs review"

[Rohitth007]

@neiljp I have split the PR as suggested, I would like to know your views on some of the pending conversations so that they can be marked resolved.

[Rohitth007]

@zulipbot remove "PR awaiting update"
@zulipbot add "PR needs review"
@neiljp I have made the suggested changes and also moved topic_display name change into a separate commit. If this feels unnecessary it can be dropped while merging.

[neiljp]

@Rohitth007 Thanks for working on this! The bugfixes are great and the no-results UI is a definite improvement 👍 As per messages in the stream I've merged the unread counts bugfix and the crash bugfix/feature (plus symbol rename), with slight changes 🎉

I've left the other commits:

• the test split ended up with a lot of duplication, and the conditional looks OK to leave as-is in comparison
• the name-change isn't too important; I'd be more interested in seeing deduplication of these panel implementations in general, a little like we have for PanelSearchBox (the user-list case would need refactoring first though)
• the search improvement with the caption reset is probably part of what we want - I've brought it up in the stream under Panel search box UI

On that basis I'm going to close this now, and I'd suggest we build on that last commit in a new PR once we settle on a UI.

[Rohitth007]

Thanks for merging this 🎉 . Having a common Panel would be brilliant, it would be easier to maintain and create new ones, but care should be taken for the user list as you said as there is no the UserView and RightColumn are mixed up. I'll look into this 👍