[Bug 48667] Device Manager is able to show multiple about dialogs

[agro1986]

Public bug reported:

This behavior is inconsistent with other apps like Gedit and Yelp. In
Gedit and Yelp, the about dialog box is also nonmodal but we can't
summon multiple about dialogs.

** Affects: hal (Ubuntu)
     Importance: Medium
         Status: Unconfirmed

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Device Manager is able to show multiple about dialogs
https://launchpad.net/bugs/48667