Show list of tags in PlacesView

Summary:
Show list of available Tags in PlacesView. In addition show "All tags" entry.
BUG: 182367

Test Plan: Assign some tags to files, force a reindex of the files and restart Dolphin to see the result

Reviewers: #dolphin, #kde_applications, alexeymin, ngraham

Reviewed By: #dolphin, #kde_applications, alexeymin, ngraham

Subscribers: kde-frameworks-devel, bruns, rkflx, mmustac, spoorun, michaelh, renatoo, anthonyfieroni, cfeck, elvisangelaccio, emmanuelp, ngraham, alexeymin, #dolphin

Tags: #dolphin, #frameworks

Maniphest Tasks: T8349

Differential Revision: https://phabricator.kde.org/D7700