Andreas Brauchli authored Aug 11, 2016 and Jens Georg committed Aug 12, 2016

The tag list underneath the thumbnails is now interactive:
* Hover over a tag to highlight it by underlining the tag text,
  unless dragging is active.
* Click a tag to switch to the tag page, as if the was selected in the
  sidebar.

Tag class changes:
* make_tag_string() now demands a sorted list of terminal tags. This
  ensures that the sort order is maintained when generating the string
  representation of the list.
  The new make_user_visible_tag_list() can be used to filter and sort a Tag
  Collection accordingly.

* Fix a bug in Tag.make_tag_string:
  A list of seen tags is kept to not show tags with the same string
  representation (but potentially different tag-path) twice. However,
  tags were never added to the list, such that it always remained empty
  and new items were thus compared against the empty list.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=717523