[client] ds: added new getFullscreen operation

As the window manager may change our mode to full screen without our
request we must ask the ds backend for the current state when we want to
toggle the mode.

client/include/app.h

@@ -55,6 +55,7 @@ void app_handleFocusEvent(bool focused);
 void app_handleCloseEvent(void);
 
 void app_setFullscreen(bool fs);
+bool app_getFullscreen(void);
 bool app_getProp(LG_DSProperty prop, void * ret);
 
 EGLDisplay app_getEGLDisplay(void);