Add mode_sigh and release to framebuffer::Session

The 'mode_sigh' function allows the client to receive notifications
about server-side display-mode changes. To respond to such a signal, the
client can use the new 'release' function, which acknowledges the mode
change at the server and frees the original framebuffer dataspace. Via a
subsequent call of 'dataspace', a framebuffer dataspace corresponding to
the new mode can be obtained. Related to issue #11.

demo/src/server/liquid_framebuffer/services.cc

@@ -196,7 +196,10 @@ namespace Framebuffer
 	{
 		public:
 
-			Genode::Dataspace_capability dataspace() { return _window_content->fb_ds_cap(); }
+			Genode::Dataspace_capability dataspace() {
+				return _window_content->fb_ds_cap(); }
+
+			void release() { }
 
 			Mode mode()
 			{
@@ -204,6 +207,8 @@ namespace Framebuffer
 				            Mode::RGB565);
 			}
 
+			void mode_sigh(Genode::Signal_context_capability) { }
+
 			void refresh(int x, int y, int w, int h) {
 				window_content()->redraw_area(x, y, w, h); }
 	};