If a theme cannot be found in the user directory, try in system
data dirs (XDG_DATA_DIRS, usually /usr/share and /usr/local/share)
Signed-off-by: Giovanni Campagna <gcampagna@src.gnome.org>
Previously null stylesheets were ignored, making it impossible to
reset the default. We still need to filter out invalid (non existing)
stylesheets, as no check is performed by shell code.
Extensions that use public Shell API are expected to work for all
stable 3.0 releases, so we skip setting the minor version. (This
means that they'll work in 3.0.0 and 3.0.1 without rebuilding).
Notably absent are auto-move-windows and windowsNavigator, that
hook deep into Shell code and could be broken by next minor release.
