xapp-status-icon.c: Refactor event handling.

- Removed popup-menu signal.  All menu handling is done within XAppStatusIcon.
- Primary and secondary menu can be set now.
- Buttons with no corresponding menu will always send the 'activate' signal
  instead.
- Middle button always sends activate.
- button-press- and button-release events are always sent regardless.

- Added more test scripts to test various possible menu configurations.
- Re-use some common code for native and fallback.

Author: mtwebster