The main window has one implementation of gUIDensity, which updates other windows, and I'm about to add another implementation in bug 1727872 because the Address Book tab doesn't get updated automatically.

Apart from the obvious duplication problem, the main window object would add a new observer for every main window that opens, and each of them would try to update all other windows. We should centralise it all somewhere in a JSM so only one observer is needed, and either figure out how to update all windows and tabs automatically, or provide a way to register for updates (like colorTracker in calViewUtils.jsm).

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/997d459216f8
Put all the UI density code in one module. r=mkmelin