Refactoring of show/hide of "busy spinner" in branch management bar

Introduction of a counter of the requests in progress :

• in IDE.java methods, increment a service counter numberOfActionsRunning
  the "busy spinner" widget listens to the counter. When 0, hide; when > 0, shows
• makes things simpler to manage, allows for nested actions calls, for parallel actions, etc.
• easy to manage error/timeout on actions launched, and still keep UI up to date

The show/hide "busy spinner" should in the end rely on notifications of counter change, and hide when <= 0.