output: Add output_update_usable_area/all_usable_areas()

output: Add output_update_usable_area/all_usable_areas()

Move the desktop_arrange_all_views() call outside layers_arrange() into
a new function, output_update_usable_area().  The new function currently
does exactly what layers_arrange() used to, but will be expanded in a
later commit.

Add output_update_all_usable_areas(), which is the same as calling
output_update_usable_area() for each output, but only calls
desktop_arrange_all_views() once.

Rebased and slightly modified by @Consolatis