Allow window matching based on the group leader's name and class (Fix bug 5721)

In Openbox 3.4 we used the group leader's WM_CLASS value when it was available.
This prevents windows in the group from overriding with a specific value,
however which is bad. More rationale can be found in
http://icculus.org/pipermail/openbox/2010-September/006790.html

Some applications (eg. Firefox) use command line flags to set the WM_CLASS
property on the group leader but do not change the property on the mapped
windows themselves. This makes matching these windows not possible in Openbox
3.5.

We resolve this by exposing the group's WM_CLASS values alongside the individual
window's values. We add _OB_APP_GROUP_NAME and _OB_APP_GROUP_CLASS properties
along with "groupname" and "classname" attributes for the rc.xml application
tag.