Update .gitignore
xsettings: Set Gtk/DecorationLayout from wm preferences

GTK+'s client-side decorations follow an XSetting to determine
which standard window buttons to show, similar to mutter's
button-layout setting in org.gnome.desktop.wm.preferences.
Keeping both mechanisms completely separate obviously means we
can end up with unwanted inconsistency, so use the GSetting
to back the XSetting on GNOME, just like we already do for
other settings (though it's a bit more involved, as the
possible values differ between mutter and GTK+).

https://bugzilla.gnome.org/show_bug.cgi?id=730629
xsettings: Explicitly handle classic mode for now

There are plans to add session-dependent defaults to GSettings
(based on the newly standardized XDG_CURRENT_DESKTOP); until
then, the WM uses a different schema for its button-layout
setting in classic mode. So for the time being, do the same
and pick the alternative schema when XDG_CURRENT_DESKTOP
indicates that we are in a classic session.
(It's not pretty, but hopefully won't be with us for too long ...)

https://bugzilla.gnome.org/show_bug.cgi?id=730629