minor cleanup of qml's selected id management

src/timeline2/view/qml/Clip.qml

@@ -244,11 +244,11 @@ Rectangle {
         cursorShape: (trimInMouseArea.drag.active || trimOutMouseArea.drag.active)? Qt.SizeHorCursor : dragProxyArea.cursorShape
         onPressed: {
             root.autoScrolling = false
+            root.mainItemId = clipRoot.clipId
             if (mouse.button == Qt.RightButton) {
                 if (timeline.selection.indexOf(clipRoot.clipId) == -1) {
                     controller.requestAddToSelection(clipRoot.clipId, true)
                 }
-                root.mainItemId = clipRoot.clipId
                 root.clipFrame = Math.round(mouse.x / timeline.scaleFactor)
                 root.showClipMenu()
             }

src/timeline2/view/qml/timeline.qml

@@ -50,6 +50,9 @@ Rectangle {
         if (dragProxy.draggedItem == itemId) {
             endDrag()
         }
+        if (itemId == mainItemId) {
+            mainItemId = -1
+        }
     }
 
     function updatePalette() {