Support 'sticky' and 'non-sticky' panel searches

[neiljp]

To use a term from #822 and related discussion, 'sticky' refers here to when state is retained, and possibly applied to other actions, and doesn't automatically reset.

Currently the side panel searches (filters) are all 'sticky', in that once one does a search, enters the list and activates on an item (including switching between streams/topics), the search remains.

The existing 'sticky' behavior can be convenient in some situations. For example, one may want to search for some topics in a stream, and retain that filtered list in order to check through other results without needing to re-enter the search/filter text.

On the contrary, there are a number of situations where searches may be preferably non-sticky:

• one can search and use enter/t, and the search/filter will revert upon showing the stream list
  • in particular, rather than scrolling down a long list to use enter/t, one can search and do so
  • this avoids needing to navigate to the stream list and use esc to clear the list
• one may wish to find one particular user via w, but after leaving the view simply want the list to revert automatically
  • again, this avoids the need to navigate back to the user list and hit esc

As discussed in #zulip-terminal > UX for stream search #T1205 we can consider the non-sticky case to be the more common case, but as above and in the discussion there, we wish to maintain the sticky case.

The current proposed solution is therefore:

• adjust current search filter keys to be upper-case (W/K), so requiring an extra modifier, but very close and simple to remember
• lower-case keys should be adjusted to clear search/filter state after activation/use/leaving-view
• search boxes should show both keys

Stream/topic list and user list may require different approaches.