Retire MPD

[Willy Tschager]

Because it's:
- Not necessary
- LDCad (and other programs) don't save the file with a MPD extension if it is an MPD
- I'm tired of the MPD vs. LDR extension argument with the OMR

The library can stay DAT since:
- We would have to recycle almost the entire library to update the type lines
- Provides a nice distinction between user edited files (LDR) and the library.

I'm fine with MPD. A quick look at the extension tells you that there will be submodels and that it is or a scene or a model say with movable subparts.

I wish LDCad had proper MPD support for import/export of submodels.

w.