Dialogs don't return focus to editor

[njx]

1. Open Extension Manager or File > Project Settings
2. Click Close or OK
3. Type something

Result: Does nothing. Focus isn't returned to the editor.

[njx]

We'll need #7493 (and possibly a wider fix) when fixing this.

[njx]

Also need to be careful that if you have nested dialogs, it actually returns focus to the previous editor.

[njx]

Reviewed, low pri to me.

[JeffryBooher]

Here is a pull request that fixed it but there was an outstanding issue which was not addressed so it was closed after several months of inactivity: #4957

[peterflynn]

@njx You could probably just lift the code from DropdownButton (_lastFocus, etc.) since that already implements the behavior I think we'd want.