There are rules that have to be applied only once, e.g. every Remember
and Apply Initially rule, as well rules that need to configure the client,
e.g. size, etc. In the best scenario the compositor would evaluate such
rules when the client is about to be mapped.
This change limits window rules only to xdg-shell clients because right
now only this protocol lets compositors to intervene in the client
initialization process. Also, it makes things a bit easier for us on the
xdg-shell protocol satisfies most of ours requirements to implement window
rules, but not all of them. If the client is about to be mapped for the
second time and its size is forced by a rule, then compositor may need
to configure it. Currently, xdg-shell protocol doesn't have any mechanism
that a client could use to notify the compositor about its intent to map.