UI: Add transition on double-click studio mode option

This adds the ability to switch to a scene by double-clicking it when in
studio mode.

(Edit by Jim: In case this change is undesired by the user, this has
been changed to be an option in general settings; disabled by default)

Closes jp9000/obs-studio#1029

UI/window-basic-main.cpp

@@ -3794,6 +3794,24 @@ void OBSBasic::on_sources_itemDoubleClicked(QListWidgetItem *witem)
 		CreatePropertiesWindow(source);
 }
 
+void OBSBasic::on_scenes_itemDoubleClicked(QListWidgetItem *witem)
+{
+	if (!witem)
+		return;
+
+	if (IsPreviewProgramMode()) {
+		bool doubleClickSwitch = config_get_bool(App()->GlobalConfig(),
+				"BasicWindow", "TransitionOnDoubleClick");
+
+		if (doubleClickSwitch) {
+			OBSScene scene = GetCurrentScene();
+
+			if (scene)
+				SetCurrentScene(scene, false, true);
+		}
+	}
+}
+
 void OBSBasic::AddSource(const char *id)
 {
 	if (id && *id) {