f58b532 implemented output-relative position saving/restoring on output
un-plugging/re-plugging. It worked as follows:
1. Store the output-relative view geometry in `view->last_placement`
(if not set) before adding/removing an output from the layout.
2. After adding/removing an output, call `view_adjust_for_layout_change()`
after the layout change to restore the output-relative view geometry
based on `view->last_placement`.
However, it didn't consider `view_adjust_for_layout_change()` being
called from other places such as `regions_reconfigure()` and
`output_update_all_usable_areas()`, causing an error message "view has
no last placement info". This can happen when a panel is mapped or
unmapped, or on Reconfigure.
This commit fixes it by changing the life cycle of
`view->last_placement`. It used to be set only before output layout
changes and cleared on user-initiated moves/resizes, but now it is set
and updated on user-initiated moves/resizes. I think this is more
intuitive, too.
12b6d05481