[Trac #583] Make sugar-dev-console a singleton

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#583: Make sugar-dev-console a singleton
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 Reporter:  marco   |       Owner:  dcbw      
     Type:  defect  |      Status:  new       
 Priority:  normal  |   Milestone:  BTest-2   
Component:  sugar   |    Keywords:  sugar-love
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 We want to be running always only one instance of the developer console.
 Here is how it could be implemented:

 * Add a dbus service with a "present" method, dbus activation would take
 care of running the executable.
 * Make the process exit when the window is closed.
 * Move the keybinding from matchbox to Shell.py. Just call the present
 method from there.

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Ticket URL: <http://dev.laptop.org/ticket/583>
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