Gui/Engine separation

- Separate OfxOverlayInteract from the GUI by adding a OverlaySupport
interface that ViewerGL and CurveWidget implement.
- Add Natron enums with conversion function from  Qt enums. This allows
to remove QKeyEvent and QMessageBox from Engine.
- Support the keyboard symbols directly by interfacing with
ofxKeySyms.h so the overlays are actually passed the expect symbols.