Launcher - Add possibility to cancel Launcher icon drag and drop by pressing ESC

Bug Description

Within most GTK applications, it is possible to cancel a drag and drop operation using "Esc". This is especially helpful when one accidentally triggers drag and drop, and wants to cancel it without having any effect of the operation.

Steps to reproduce:

1. Drag an item from the launcher
2. Hit "Esc"

Current incorrect behavour:

- The item does not respond to "Esc" being hit, it is not possible to cancel the drag and drop operation.

-----------------------------
Desired resolution:

- When Esc is pressed while a Launcher icon drag is in progress, the Launcher icon should go back to its original position.

- The icon should move back to the Launcher in an animated fashion, with the icon sliding back to it's position in the Launcher when Esc is pressed.

Update: Unity 6.4 is revision 2632, so this is actually fix committed for Unity 6.6.

This bug was fixed in the package unity - 6.6.0-0ubuntu1

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unity (6.6.0-0ubuntu1) quantal-proposed; urgency=low

* New upstream release.
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- LauncherDragWindow: cancel drag on window mapped/unmapped
(LP: #1044723)
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- Fixed ability to delete glib::Source wrapper during its callback
(LP: #1044823)
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