Reorganize the existing code which parses the -st-shadow property
to allow parsing different shadow properties and add support for
the text-shadow property.
https://bugzilla.gnome.org/show_bug.cgi?id=624384
Add basic support for background-position, which only supports absolute
values.
Also don't require an unit to be specified for 0 (because the unit does not
really matter here 0 is 0 regardless of the unit).
https://bugzilla.gnome.org/show_bug.cgi?id=624375
The idea behind this move is that we have a lot more control over
rendering if StWidget isn't a big pile of actors, and things are
more efficient.
https://bugzilla.gnome.org/show_bug.cgi?id=607500
