display: separate input/non-input events handling in the event callback

In order to make the XI2 handling easier on us in the future, we now
split input events from non-input events. This will allow one code path
to use XIEvent, and the other to use XEvent in the future. This commit
has involved plenty of indenting changes, so it's better seen with
git diff -b or &ignorews=1

https://bugzilla.gnome.org/show_bug.cgi?id=688779