Implement for barLevel an overdrive area. This is a zone represented via a
different styling to indicate that you are bypassing the normal zone of
a given level, without reaching yet the maximum limit.
https://bugzilla.gnome.org/show_bug.cgi?id=790280.
Ensure that both barLevel and slider can support a higher maxValue than 1
and computes various positions based on it.
It defaults to 1 if not set.
https://bugzilla.gnome.org/show_bug.cgi?id=790280.
Reuse the BarLevel class to get similar drawing behavior as Slider.
Rename theme css impacted properties and ensure that the osdWindow
remains accessible.
Ensure we don't force setting a custom border color like on the OSD.
https://bugzilla.gnome.org/show_bug.cgi?id=790280.
