LibGUI: Don't restart AbstractView drag on mousemove after escape key

Previously, if a drag operation was aborted by pressing the escape key
(handled by WindowServer), the drag would immediately restart if you
moved the mouse cursor before releasing the mouse button.

Userland/Libraries/LibGUI/AbstractView.cpp

@@ -308,6 +308,7 @@ void AbstractView::mousemove_event(MouseEvent& event)
         break;
     case DragOperation::Outcome::Cancelled:
         dbgln("Drag was cancelled!");
+        m_might_drag = false;
         break;
     default:
         VERIFY_NOT_REACHED();