Several fixes to browseMode and base TreeInterceptor to fix regressions when focusing an Excel worksheet. Fixes #5238.
* Base TreeInterceptor: remove a possibly accidentally committed implementation of event_gainFocus. It is simply not needed, and was causing Excel focus not to work when alt+tabbing back. Fixes #5238
* BrowseModeDocument._activatePosition: make the info argument optional, and if not provided make it create a TextInfo for position_caret.
* BrowseMode: move script_activatePosition from BrowseModeDocument into base BrowseMode as that is where it is bound. Also change its implementation to just call _activatePosition and let that method handle TextInfos etc entirely itself.