Unlink a clip if its linked clip is removed

src/Gui/timeline/Clip.qml

@@ -128,6 +128,9 @@ Rectangle {
         Drag.drop();
         selected = false;
 
+        if ( linkedClip )
+            findClipItem( linkedClip ).linkedClip = "";
+
         for ( var i = 0; i < allClips.length; ++i ) {
             if ( allClips[i] === clip ) {
                 allClips.splice( i, 1 );