If a group's sub-item is selected in the list box and you select the
group via the preview, the sub-item would be deselected, but that
deselection would not be applied to the sub-item in the list box despite
being deselected.
This commit adds the ability to select a background color for a
scene-item, whether it's a custom color or one of eight presets.
As this is an initial implementation, it lacks theme customizability,
and it also lacks the ability for the user to set their own preset
colors, so only the hard-coded 8 are available.
Because sub-items of groups can no longer reroute their signals to the
parent scenes of groups, delegates removal handling to SourceTreeItem
instead of OBSBasic.
