[Enhancement] Add multi-selection in Repositories list (Welcome Page)

[goran-w]

The context-menu for selected entry in the repo-list has commands Move to Another Group and Delete, but this quickly becomes a chore if we have lots of repos and want to bring some relevant grouping to them or cleanup left-overs etc.

The simplest improvement to this situation would be to allow multi-selection in the list, and to allow these two commands to be performed on multiple items at once.

NOTE: Drag-drop does not have to work for multi-selection (but it would be nice if it did). As soon as there exists ANY way of moving/deleting multiple repo-items at once, I'd say this ticket can be closed...

[goran-w]

We have a LOT of repos, and trying to get a good overview using Groups in the Repositories tab/list is unnecessarily tedious due to the lack of multi-selecting. Being able to Group/Delete items in bulk would solve that issue elegantly.

Also, to make better use of the Groups, they should be displayed also in the 'Switch active tab' popup - see #1932.