UI: Disable source copy if no sources are selected

This commit addresses Mantis Bug 993. If no source was selected, the
"Copy" context menu item was enabled, but selecting it would do nothing.
This commit disables that menu item when no sources are selected.

UI/forms/OBSBasic.ui

@@ -182,6 +182,9 @@
      <addaction name="actionScaleOutput"/>
     </widget>
     <action name="actionCopySource">
+     <property name="enabled">
+      <bool>false</bool>
+     </property>
      <property name="text">
       <string>Copy</string>
      </property>

UI/window-basic-main.cpp

@@ -2663,6 +2663,8 @@ void OBSBasic::SceneItemSelected(void *data, calldata_t *params)
 	QMetaObject::invokeMethod(window, "SelectSceneItem",
 			Q_ARG(OBSScene, scene), Q_ARG(OBSSceneItem, item),
 			Q_ARG(bool, true));
+
+	window->ui->actionCopySource->setEnabled(true);
 }
 
 void OBSBasic::SceneItemDeselected(void *data, calldata_t *params)
@@ -2675,6 +2677,8 @@ void OBSBasic::SceneItemDeselected(void *data, calldata_t *params)
 	QMetaObject::invokeMethod(window, "SelectSceneItem",
 			Q_ARG(OBSScene, scene), Q_ARG(OBSSceneItem, item),
 			Q_ARG(bool, false));
+
+	window->ui->actionCopySource->setEnabled(false);
 }
 
 void OBSBasic::SourceLoaded(void *data, obs_source_t *source)