Fix FlipView / WorkingSet / SplitViewSameDoc integration issues

[petetnt]

This PR is a continuation to #12060 and is similarly targeted to @swmitra's BugFixesFor1.6Beta branch instead of the master.

This PR fixes following issues:

Dirty doc working set flip issue

Issue 3( Minor):

As I have mentioned earlier in my same doc split PR , the file which was not present in working set , gets added to the working set after flip. The fix is not trivial in this case , I tried commenting the line where we are triggering working set event but looks like that results in stale mrulist entry.

We check in Pane.js#L585 if the current file being flipped is actually on the working set: if it isn't, we destroy the view and return as resolved instead of adding it to the other Panes _viewList

Flip the behavior of focused-editor-after-flip

As discussed in #12060 (comment), this commit essentially flips the following commit around: petetnt@8bc2845

This way the focused editor after flipping any view is the same one that was focused before the flip.

Prevent splicing the wrong file when the file isn't on the working set

#12060 (comment)

This issue was caused by the same problem than the previous one.

Unify handling of flipping and drag & dropping across working set

#12060 (comment)

By closing duplicate entries before flipping the view (if it already is open on the other pane) we make sure that D&D and FlipView scenarios handle the same way: currently the pane the file gets dragged/flipped to stays open_[1]_. This also fixes issues where clicking the FlipView button duplicates the current view instead of just moving it to another one (when the same file had previously been opened and closed in the other pane).

/ping @swmitra 📝

This PR together with #12060 still needs some extensive unit tests, but I think those are easier to add in bulk when we get the baseline functionality fixed.

[1] This would need some kind of merge functionality as discussed in #12060 (comment), but I don't think it's possible to land in 1.6.

[swmitra]

@petetnt Sorry for late reply. I have tested most of the scenarios along with the reported scenarios for the issues. It looks fine to me. Great work as always! 👍

Merging this PR with the bug fixes branch swmitra/BugFixesFor1.6Beta

Will continue with some more testing.

[petetnt]

👍