feature: close split view automatically

[zaggino]

steps:

1. open preferences file
2. split view opens
3. click to close the brackets.json in the right pane, nothing else is remaining in the right pane

i believe, that the split view should now close automatically, given it was also automatically opened

[petetnt]

Makes sense in the preferences scenario, but in general I think that the preferences split view should be more isolated anyway. Auto-closing the panel when it's possible to modify the views while the preferences are open is a bit tricky too. Consider something like this:

1. Open preferences file
2. Split view opens
3. Drag another file to right panel
4. Close brackets.json -> Right view should stay open right?

But what if

1. Drag the file back to left panel
2. Close brackets.json - nothing stays in the right panel but it shouldn't auto close as it has been "dirty" - right?

[zaggino]

Well, I'd implement it in a way that if split-view was automatically opened it also should automatically close. If it was opened manually, it should never auto-close. I don't really like split view and how it's forcing itself anytime I want to change something in preferences.

[petetnt]

This was added in #11749 right?