Application detect on second screen

[FestiveMongoose]

Dockx version: 0.90.3-6
OS: Arch Linux
Kernel: 3.12.2-1-ARCH x86_64 GNU/Linux
WM: Gnome 3.10.1

Description:
When using dual screens, and dockbarx is on the first screen (left, primary), moving applications such as remmina to the second screen and spawning/closing them on the second screen results in dockx inability to remove the closed windows from its list of open windows.

To reproduce:
- open remmina
- move remmina main window to second screen
- open a remmina connection profile
- close all remmina windows

* RESULT*
you will see dockx reporting two open windows for remmina, one being "connecting to 'profile_name'..." other one being the name of the profile.

Debugging:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/dockbarx/dockbar.py", line 475, in __on_window_closed
    dock.remove_window(window)
  File "/usr/bin/dockx", line 930, in remove_window
    del self.border_distances[window]
  File "/usr/lib/python2.7/weakref.py", line 253, in __delitem__
    del self.data[ref(key)]
KeyError: <weakref at 0x212bdb8; to 'wnck.Window' at 0x21390a0>