- Tag logic/management is now in KoResourceTaggingInterface
- KoTagChooserWidget is a custom widget that contains
  the combobox used for tag selection as well as a custom
  QToolButton to interface with addition/deletion operations
  for it.
- KoTagFilterWidget is a custom widget that contains a line
  edit, to enter filter terms, and a save button to save
  any changes done to a selected tag set.

Improve robustness of a lot of tagging related code by passing
around concrete parameters to functions that alter the content of tag
sets instead of relying on variables that might be out of synch.

REVIEW: 112080