NEWT AWTAdapter: Simplify event processing / Window.enqueueEvent(..): Add API doc

Simplify event processing of NEWT's AWTAdapter:
  - enqueueEvent -> processEvent
  - add return value (new enum) determinating action

Using processEvent(..) now reduces manual invocation of NEWT Window.enqeueEvent(..).
Bug 741 HiDPI: Add new NativeSurfaceHolder interface to GLDrawable and NativeWindow; [AWT|SWT]NewtEventFactory use NativeSurfaceHolder as source, fixes pixel unit conversion

- Add new NativeSurfaceHolder interface to GLDrawable and NativeWindow, allowing NativeSurface access (pixel unit conversion)
  A NativeSurfaceHolder is e.g.:
    - NativeWindow (is-a)
      - NEWT [GL]Window

    - GLDrawable (has-a)
      - [AWT|SWT]GLCanvas

- [AWT|SWT]NewtEventFactory use NativeSurfaceHolder as source, fixes pixel unit conversion