Retire MPD

[Orion Pobursky]

LDCad doesn't care about the extension, only header, content and as a fallback the location (library or not) are used.

But why would you want to retire the extension?

I don't really see an advantage, unless we drop .dat too and use .ldr for everything.

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.