shell: don't abuse link_layer for keeping track of input panel surfaces

shell: don't abuse link_layer for keeping track of input panel surfaces

Input panel surfaces were kept in a list by using layer_link of
weston_surface. This was pretty hacky and resulted in the bug that
an input panel surface was not removed from the list if it was unmapped
at the time of destruction.

This patch wraps the surface in a new input_panel_surface struct and
properly handles destruction with a signal listener.