tweakview: Update decorations on settings changes

It is a bit ironic that Tweak Tool allows users to tweak the
titlebar decorations, but ignores the setting (at least until
a restart). So listen for change notifications and update
decorations accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=737531
windows: Stop overriding the Gtk/DecorationLayout xsetting

gnome-settings-daemon monitors the button-layout setting and uses
it to set Gtk/Decoration - unless there's an explicit override.
In the best case, the override matches the setting and it doesn't
make any difference, in all other cases it causes client- and
server-side decorations to go out of sync.

https://bugzilla.gnome.org/show_bug.cgi?id=737533