commit -- updated most of abi/docs; created a few new ones.

[Jeff Hostetler]

i've updated most of the documentation in abi/docs
and i created a few new documents.

AbiSource_DialogFramework.abw           -- new, TR on adding dialogs
AbiSource_ProgrammerGuidelines.abw      -- minor updates
AbiWord_DocumentFormat.abw              -- major revamp, now matches 0.5.0
AbiWord_FileImport.abw                  -- new, TR on writing importers
AbiWord_SourceOverview.abw              -- fleshed-out empty sections

design.txt                              -- nuked it, it was very, very old

enjoy,
jeff