[Gtkradiant] Filter api

[Martin Gerhardy]

Hi,

in revision 208 i've commited the filter api changes. But i still don't
get the hang on the mainframe.cpp stuff (at
http://zerowing.idsoftware.com/viewcvs/radiant/GtkRadiant/branches/ZeroRadiant/radiant/mainframe.cpp?view=diff&rev=208&p1=GtkRadiant/branches/ZeroRadiant/radiant/mainframe.cpp&r1=207&p2=GtkRadiant/branches/ZeroRadiant/radiant/mainframe.cpp&r2=208 )

Why do they have to be removed and readded? This will of course remove
all the plugin filters added.

Also the quake2 filters doesn't really work yet - because the getFlags
for the (pseudo-)shaders doesn't seam to return the content or surface
flags. I have to look into it again. Except that the filter api
__should__ work. Feedback is welcome.

Greetings
Martin