Initial import of !SrcEdit, exported from the old SrcFiler repository and given a modern build structure.

Detail:
  I'm not convinced these are actually the most recent sources. They didn't
  build as-is against a "modern" RISC_OSLib, and I had to substitute the
  !Run/!Boot/Messages etc from the C/C++ tools release to make it work.
  There are still some issues - some menus are incorrectly created, and one
  source file invokes a compiler bug.
Admin:
  Not particularly thoroughly tested.

Version 0.01. Tagged as 'SrcEdit-0_01'