gui: TagEditorWidget: adding line for tags adding
gui: TagEditor: Model: access to used tags. Emits signal when changed
gui: TagEditor: fill combo with tag names with proper list of names
gui: TagEditor: enabling/disabling gui elements depeding on photos selection
gui: TagEditor: fixing parentheses
gui: TagEditor: Model: allow tags to be added
gui: TagEditor: add tag when 'add' button is pressed
Merge branch 'development' - fix for issue #81