We have enough info to display the iwad box now!

- PyResourceFile.numlumps() so that one does not have to look at the
names directories to find the number of lumps a wad contains.
- Python: Manually control the GC.
- Added a matches() method to IWad to check if the passed ResourceFile
contains the required lumps
- Main: Scan and match the required IWads against ones we located, then
display the IWad picker window with the one we found and store the
result. We don't actually do much with it yet but it's quite a step
forward.