More functionalities and some bugfixes

New functionalities :

• Handle loading of multiple folders or multiple plugins whenever the
  user add one during editing.

• Load plugin now open folder containing last generated plugin

• Added Merge action

Removal

Removed QLCDNumber displaying number of animations found -> Useless

Bugfixes:

• [CRITICAL] Convert AnimTreeItem seen as TreeItem back to correct class.
• [CRITICAL] Insert parent now supports multi-level selection