Add .xsession installation base code.

This isn't turned on yet (the testcase just tries to run the function
without exceptions), but this would be the base for a feature to install
the new sample .xsession file for the "Custom session" login option for
most login managers.

The base .xsession was graciously provided by Michael Janses (of
build-tool fame). I've simplified it a bit for kdesrc-build usage and
tried to make it compatible with less featureful shells. The eventual
idea is that it can be used with kdesrc-build-set and/or kdesrc-build so
that you can run the setup script, kdesrc-build, and be able to login to
your new KDE install.