dnd: Add a drag-cancelled signal

This lets us start the workspace zoom out animation right when the
snapback starts, rather than waiting for the snapback to finish.

https://bugzilla.gnome.org/show_bug.cgi?id=643786

js/ui/dnd.js

@@ -498,6 +498,7 @@ _Draggable.prototype = {
     },
 
     _cancelDrag: function(eventTime) {
+        this.emit('drag-cancelled', eventTime);
         this._dragInProgress = false;
         let [snapBackX, snapBackY, snapBackScale] = this._getRestoreLocation();
 

js/ui/overview.js

@@ -481,6 +481,10 @@ Overview.prototype = {
         this.emit('window-drag-begin');
     },
 
+    cancelledWindowDrag: function(source) {
+        this.emit('window-drag-cancelled');
+    },
+
     endWindowDrag: function(source) {
         this.emit('window-drag-end');
     },

js/ui/workspace.js

@@ -125,6 +125,7 @@ WindowClone.prototype = {
                                               dragActorMaxSize: WINDOW_DND_SIZE,
                                               dragActorOpacity: DRAGGING_WINDOW_OPACITY });
         this._draggable.connect('drag-begin', Lang.bind(this, this._onDragBegin));
+        this._draggable.connect('drag-cancelled', Lang.bind(this, this._onDragCancelled));
         this._draggable.connect('drag-end', Lang.bind(this, this._onDragEnd));
         this.inDrag = false;
 
@@ -292,6 +293,10 @@ WindowClone.prototype = {
         this.emit('drag-begin');
     },
 
+    _onDragCancelled : function (draggable, time) {
+        this.emit('drag-cancelled');
+    },
+
     _onDragEnd : function (draggable, time, snapback) {
         this.inDrag = false;
 
@@ -1231,6 +1236,10 @@ Workspace.prototype = {
                           Main.overview.beginWindowDrag();
                           overlay.hide();
                       }));
+        clone.connect('drag-cancelled',
+                      Lang.bind(this, function(clone) {
+                          Main.overview.cancelledWindowDrag();
+                      }));
         clone.connect('drag-end',
                       Lang.bind(this, function(clone) {
                           Main.overview.endWindowDrag();