Restore selection when editor is refocused

When the editor is focused, the selection goes back to the start of
the document. This was remedied in the .focus() method, but not in
response to native focus events, so when external code blurred then
refocused the editor, the selection would move to the top.

This broke section editing on wikis where ULS is installed: the
selection would be initialized at the start of the section, but then
ULS would load and blur the documentNode (by focusing the pasteTarget)
and then focus it again, so the selection would move to the top.

Instead of restoring the selection only in .focus(), restore it in
response to focus events on the documentNode. When this is done,
saving and restoring the scrollTop is no longer needed.

Bug: 56651
Change-Id: I14700174ee092f9b208215d31a7d1871078a89bf
Merge "Restore selection when editor is refocused"