Allow clicking on top panel to return to home screen

Allow clicking on top panel to return to home screen

This is a work-around solution to allow us to return to the home screen in wayland since
there's no home button.

1. Defined new 'show_homescreen' request in desktop shell protocol.
2. Added handling in weston-shell to handle when show_homesreen is called, it raises
the surface belongs to the home screen (which is the org.tizen.menu-screen).
3. Registered click event when you click on the top launcher panel in the desktop shell,
it will return to the home screen, since the top launcher panel is always present in the
desktop shell client.

Signed-off-by: Jimmy Huang <jimmy.huang@intel.com>