Improves the ux of the new grouped GitLens view

Renames "Close" to "Regroup with GitLens View" on detached views
Renames "Move to Separate View" to "Detach <view> View"
Adds new "Regroup <view> View" action to the grouped view's `...` menu
Adds keyboard shortcuts (1-9) for quick view switching in grouped mode
Ensures views are focused when switching between grouped modes

Author: eamodio