Currently the view destruction sequence is started as soon as a view
is unmapped. However, this is incorrect as a client may map the view
again instead of destroying it.
Instead, only start the view destruction sequence when the underlying
xdg toplevel or xwayland surface is destroyed.
There is now a single iter() function which accepts a filter and context
allowing users of the api to filter the views in any arbitrary way. This
change allowed for a good amount of code cleanup, and this commit also
ensures that the correct properties are checked in each case, including
the new View.destroying field added in the previous commit. This fixes
at least one crash involving switching focus to a destroying view.
This simplifies the handling of the current/pending tags and will be
used in the future for atomic layout updates involving layer surface
exclusive zones.
The state struct holds all of the state that is double-buffered and
applied through transactions. This more explicit handling simplifies
much of the code, and will allow for easier implementation of new
feature such as fullscreen.
