Added support for layered maps. This support is still very

Added support for layered maps.  This support is still very
experimental, and the potential for crashes is rather large.  Still,
there is demand for it, so I'm posting it early.  By default, layers are
disabled, so it should not affect normal users.

git-svn-id: svn+ssh://garage/var/lib/gforge/svnroot/maemo-mapper/trunk@211 6c538b50-5814-0410-93ad-8bdf4c0149d1