Drag & drop implementation at GHOST level (only OSX for now)

The dragging sequence is performed in four phases:
 
- Start sequence (GHOST_kEventDraggingEntered) that tells a drag'n'drop operation has started. Already gives the object data type, and the entering mouse location

- Update mouse position (GHOST_kEventDraggingUpdated) sent upon each mouse move until the drag'n'drop operation stops, to give the updated mouse position.
Useful to highlight a potential destination, and update the status (through GHOST_setAcceptDragOperation) telling if the object can be dropped at the current cursor position.

- Abort drag'n'drop sequence (GHOST_kEventDraggingExited) sent when the user moved the mouse outside the window.

- Send the dropped data (GHOST_kEventDraggingDropDone)

- Outside of the normal sequence, dropped data can be sent (GHOST_kEventDraggingDropOnIcon). This can happen when the user drops an object on the application icon. (Also used in OSX to pass the filename of the document the user doubled-clicked in the finder)

Note that the event handler is responsible for freeing the received data.
And the mouse position is sent directly in blender client coordinates (y=0 at bottom)

The GHOST_setAcceptDragOperation(TRUE) call must be placed before the user drops the object for it to be accepted.

Current handled data types :
- Text string
- Array of filenames (full paths)
- Bitmap image (not implemented yet)

intern/ghost/intern/GHOST_System.cpp

@@ -54,6 +54,7 @@
 GHOST_System::GHOST_System()
 : m_displayManager(0), m_timerManager(0), m_windowManager(0), m_eventManager(0), m_ndofManager(0)
 {
+	m_canAcceptDragOperation = false;
 }
 
 
@@ -275,6 +276,15 @@ GHOST_TSuccess GHOST_System::getButtonState(GHOST_TButtonMask mask, bool& isDown
 	return success;
 }
 
+void GHOST_System::setAcceptDragOperation(bool canAccept)
+{
+	m_canAcceptDragOperation = canAccept;
+}
+
+bool GHOST_System::canAcceptDragOperation() const
+{
+	return m_canAcceptDragOperation;
+}
 
 GHOST_TSuccess GHOST_System::init()
 {