Improve the use of Groups in the Repositories list (on Welcome page)

[goran-w]

• Alternative 'folder-tree' layout, forfeiting Groups?
• Specify (optional) Group already when Cloning, to avoid all cloned repos ending up in 'root'?
• (Also specify optional 'root'-Group when scanning folder?)
• Show the Groups in Tabs popup (Ctrl+P) - [Enhancement] In the 'Switch active tab' popup, show Groups under Open Repositories sourcegit-scm/sourcegit#1932