Move the input_panel interface from desktop-shell to input-method (since
it is not really tied to desktop-shell).
Add an input_panel_surface interface like wl_shell_surface to make it
easier to extend it. Also add a parameter to the set_toplevel request to
be able to specify where to show an input panel surface on the screen.
Signed-off-by: Jan Arne Petersen <jpetersen@openismus.com>