Integrate the editor with Descent3 source

[JeodC]

Properly integrate D3Edit or the internal editor with Descent 3 source and add compile time options to ignore or build the editor along with D3. Possibly consolidate duplicate dependencies and libraries used.

[JeodC]

There is some uncertainty in the community. I have seen a few opinions that the developer editor in /legacy/ is superior due to the use of hardware rendering.