make sure the messagelist only shows selected folders

When the folder tree is filtered and the currently shown folder is
now no longer in the folder tree, the automatic deselection did not work
because the hidden folders were set to not-selectable and not-enabled,
but QItemSelection can only deselect selectable and enabled items.
This led to the problem that we saw the old selected folder and any
newly selected folder intermixed in the messagelist.

BUG: 287685
BUG: 314457