[openbox] DockApp detection

[Brad Jorsch]

As you may know, dockapps are detected by the WM_HINTS initial_state =
WithdrawnState when the window is first mapped. And gtk+ doesn't allow
this to be set, see https://bugzilla.gnome.org/show_bug.cgi?id=139322.
In Window Maker, we recently decided to treat apps with the WM_CLASS
res_class = "DockApp" as if they had initial_state = WithdrawnState to
allow dockapp devs to create gtk+ dockapps without hacking around the
toolkit. You might want to take the idea too.