MetaShapedTexture: don't include private headers in public ones

Private headers are not installed, so they can't be referenced
from public ones.

https://bugzilla.gnome.org/show_bug.cgi?id=705497
build: make wayland support unconditional

In the wayland branch of mutter, we want to build a wayland version
of the mutter libraries, and that's much easier if we just build
wayland support unconditionally.
The define is kept to avoid a huge diff, but should be removed
in a later patch.
Also, wayland support can still be disable at runtime, by
launching mutter without the --nested switch.

https://bugzilla.gnome.org/show_bug.cgi?id=705497
Remove HAVE_WAYLAND ifdefs

Wayland support is always enabled now.

https://bugzilla.gnome.org/show_bug.cgi?id=705497
Remove files no one cares about anymore

mutter-plugins.pc has been replaced by libmutter.pc, and the
wm properties in the control center are long gone.

https://bugzilla.gnome.org/show_bug.cgi?id=705497
wayland: make parallel installable with regular mutter

Modify all visible instances of mutter with mutter-wayland
(libraries, folders, pkgconfig, etc.), so that the wayland
branch can be installed alongside the usual X11 mutter.

https://bugzilla.gnome.org/show_bug.cgi?id=705497