Show update notification icon on Extension Manager "Installed" tab

Show an icon in the Extension Manager whenever updates to installed extensions are available: a red dot with counter overlaid on the Installed tab's icon.  This should help improve the visibility of extension updates now that the Installed tab isn't selected by default.

In the long run of course we'd like a way to filter the view down to _only_ extensions with updates available, and we'd also like update notifications to be pushed to the main UI just as they are for updates to Brackets itself... but this is an incremental, simpler step in that direction.

Screenshot:
![extman notification](https://f.cloud.github.com/assets/1197510/1463057/a38eec7e-451a-11e3-9122-d5c924d7ff5f.png)

(@larz0 Please let me know if there are any visual tweaks you'd suggest)