WIP: support setting the color bit depth (deep color)

Depends on libkscreen!40 (closed)

I want to still replace the combobox with a slider, and leave out unsupported bit depths. Is there any way to set custom slider step distances, or to set a table that converts slider values into custom values and back?

There is also the question whether or not the values should be shown at all, or if we should just advertise "deep color" as a checkbox that upgrades the bit depth to 10, to make things simpler. cc @teams/vdg

I don't think anyone actually wants to set their bit depth down to 5 (it's really ugly), and if we can figure out that a panel supports more than 10 bits (should be possible through the EDID) then that should probably be set automatically instead of 10.