Introduce a new signal "groupsChanged"

Introduce a new signal "groupsChanged"

Sometimes when items are renamed, the order of the items in the
directory is not affected, but the groups still change (simple example:
with files a, b, c, e, rename "c" to "d"). At the moment, we always emit
the itemsMoved signal in such a case to make sure that the view is
updated. However, it would be preferable if this signal was not emitted
because it can trigger some quite expensive operations which are not
needed at all.

This commit introduces a new signal groupsChanged and modifies
KFileItemModel and KItemListView such that these classes make use of it.
Some unit tests for the new functionality are included as well.

Thanks to Emmanuel Pescosta for finding a latent bug in the code which
was triggered by this change and fixed in
998954db6d53999dfa75d380cbb4ca3111589f66.

REVIEW: 111808