Bug1092967 - Gallery app to follow text selection pattern r=djf

According to the docs on mdn, -moz-user-select is inherited, so it is not necessary to set this on *. Better, I think to just specify this value on the top html element.  Then, if we need to override it on some other element, the chidren of that element will properly inherit the overridden value.

I'm coming from a theoretical perspective here. If other apps are doing this with the * selector or if there is some practical reason for doing it that way, just let me know.