tablet-shell: come back to life

tablet-shell: come back to life

A couple of fixes were made: Weston tablet-shell needed to use weston_layer,
so the compositor could rebuild the surface list correctly when repainting;
homescreen and locking are using the widget + window abstration of toytoolkit;
and widget_set_redraw_handler are being set for widgets redraw.

Also, it was given some basic meaning for lockscreen_button_handler, which
was completely disabled before. As a clean up, I updated the global listener
mechanism on tablet-shell client, using the regular way of registering a
handler instead wl_display_roundtrip -> wl_display_get_global.

Switcher still without code to proper work and the same for tablet-shell
clients, which are not launched.

krh: Edited to not scale down homescreen icons, use new load_cairo_surface()
for image loading.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>