Drop gtk-update-icon-cache complications

gtk-update-icon-cache is no longer used at build time, so a lot
of the complicated machinery we have around that (conditional
build, cross build, etc) are no longer required.
Drop a few more leftovers

The GTK_FILE_SYSTEM_ENABLE_UNSUPPORTED define is not used anymore,
and we don't install a gtk.css file for Raleigh, so no need to
uninstall one either.
Rearrange Makefile some more

Move the introspection parts all together, drop several unused
variables and targets.
Move print backend list to configure.ac

We already juggle the print backends here, might as well
produce the list.
Move print preview command to gtksettings.c

Having this in the source seems more appropriate.