wpa_gui-qt4: add system tray support

wpa_gui-qt4: add system tray support

Add system tray icon support to wpa_gui-qt4. The tray icon remains quiet
when the main dialog is visible, so it should not cause too much pain for
more conservative users of wpa_gui. The addition involves the following
changes:

* when closing wpa_gui via window manager close box, wpa_gui close event is
  ignored and it is minimised to system tray. A status message is displayed
  (or popup dialog box if tray messages are not supported) to provide a
  visual hint that the program is still running in the background.
* add File->Exit slot handler to facilitate application quit from main
  dialog
* provide a context menu with a short list of useful actions
* show/hide main dialog when icon is triggered (single click)
* ensure main dialog is visible when event handler or scan results is
  chosen from tray icon context menu
* show tray messages on connected and disconnected events, display a status
  message a few seconds after connected events

Signed-off-by: Kel Modderman <kel@otaku42.de>