ES6 finally adds standard class syntax to the language, so we can
replace our custom Lang.Class framework with the new syntax. Any
classes that inherit from GObject will need special treatment,
so limit the port to regular javascript classes for now.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/361
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.
