icon-info: remove code for unused .icon file functionality

GTK+ deprecated the functionality from GtkIconInfo, and will now always
return empty data. We can then just remove all the code related to that
feature.
view: optimize real_update_menus() a bit

Fetching the selection is expensive, and this is alreay a hot code path;
don't fetch it more than once.
view: coding style fix
canvas-container: remove unused use_drop_shadows property