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/CMakeLists.txt

@@ -236,6 +236,7 @@ set(obs_UI
 	forms/AutoConfigVideoPage.ui
 	forms/AutoConfigStreamPage.ui
 	forms/AutoConfigTestPage.ui
+	forms/ColorSelect.ui
 	forms/OBSLicenseAgreement.ui
 	forms/OBSLogReply.ui
 	forms/OBSBasic.ui