- do not resolve the backdrop texture to a texture ID at load time. This will allow custom menu classes to use this info differently.
- added a new ZSDF userstring property to dialog pages to give mods more means for customization.
- allow overriding the conversation menu class both globally through MAPINFO and per conversation in ZSDF.
Changed a description in usdf_zdoom.txt to be more truthful. It's not just one new field anymore.
(I really should learn to read these things before changing them.)
