wayland: Clean up naming of GdkWaylandDisplay pointers

wayland: Clean up naming of GdkWaylandDisplay pointers

The naming of pointers to GdkWaylandDisplay's were inconsistent.
Running the following commands in gtk+/gdk/wayland illustrate the
inconsistency:

$ grep -r '\<display_wayland\>' *.[ch] | wc -l
195
$ grep -r '\<wayland_display\>' *.[ch] | wc -l
81

This patch renames all occurrences of "wayland_display" to
"display_wayland". This is also consistent with naming in the X11
backend. A couple of whitespace changes were done as well in places
where the rename was already done, that added line breaks to long lines
that stood out.

https://bugzilla.gnome.org/show_bug.cgi?id=765474