Apart from changng the name to mildenhall-compositor in both the
configure tree and the debian packaging this also:
  * Drops the dbg package, we use dbgsym now
  * No longer try and divert the metacity themes, these aren't
    used/installed anymore

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>

Differential Revision: https://phabricator.apertis.org/D4341