Add --qss parameter to load a custom stylesheet

Because there doesn't seem to be sane way to hijack Qt's -stylesheet parameter,
we introduce our own. This also allows us to add incompatible extensions to QSS,
because we can remove them before handing over the pre-parsed stylesheet to QApplication.
Note that -stylesheet is now ignored.

The style engine now loads $datadir/default.qss, $configdir/custom.qss and --qss "file.qss"
in that order. Later block declarations override earlier ones, if they are identical.
That way, you can override parts of the system-installed default.qss in your own configdir or
on the command line.