Priorization of theme merging and merging engine settings

[Owen Taylor <otaylor redhat com>]

http://bugzilla.gnome.org/show_bug.cgi?id=70205

RC styles bindings are currently merged ordered by:

 1) Type (widget, widget_class, class)
 2) Priority specified in RC file
 3) ordering in file (later wins)

To make specified priorities meaningful, I think we 
need to reverse 1) and 2). This isn't going to be
easy to implement, but probably needs to be done. 
I doubt it will cause problems for anybody.

Also, there is no way to "unset" the engine back
to the default in a merged RC style; we might want
to support engine "" {} to mean this.

Regards,
                                        Owen