UI: Add Color Coding to Source Tree Widget

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.

UI/data/locale/en-US.ini

@@ -850,3 +850,7 @@ FinalScene.Text="There needs to be at least one scene."
 NoSources.Title="No Sources"
 NoSources.Text="It looks like you haven't added any video sources yet, so you will only be outputting a blank screen. Are you sure you want to do this?"
 NoSources.Text.AddSource="You can add sources by clicking the + icon under the Sources box in the main window at any time."
+
+# Scene item color selection
+ChangeBG="Set Color"
+CustomColor="Custom Color"