gvle: add the GlobalPlugin system

In order to customize deeply the gvle GUI, a GlobalPlugin class has
been added. It does allow to add widget to the GUI.
- Gvle has been enhanced to check fo each global plugin availabe and
  to run them.
- The PluginFactory has been changed in order to keep during all the
  session the already loaded plugin available, insteed of relaxing the
  ressources as soon as a plugin has been used once.