frame: annotate the @label params with (allow-none)
headerbar: Remove hpadding and vpadding properties

We really want these to be set by the theme, not programmatically.

https://bugzilla.gnome.org/show_bug.cgi?id=706929
[headerbar] Do not hardcode title and subtititle style

Let the css theme define them

https://bugzilla.gnome.org/show_bug.cgi?id=707051