Include the X11 headers directly in files that used to include those
wrappers, and add the Pixel typedef to files that use it.

prex.h included headers that a file including prex.h might not need
at all, so when replacing prex.h and postx.h includes, try including
only X11 headers that are actually needed in their place.

Also fix saldisp.hxx and i18n_xkb.cxx relying on indirectly included
X11 headers.

Change-Id: If0c30c7facc28db7f2e410b5e6f1d52d8c7cdfd3
Reviewed-on: https://gerrit.libreoffice.org/22132Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>