Add start of parser notes

Add notes on the current parser, and start defining how the commands parse
their own information (in EBNF notation).
More fleshing out of EBNF of commands
Parsing:  Define commands B-C

This loosely fleshes out EBNF for commands B-C.
Parsing:  Define commands D-E

This loosely fleshes out EBNF for commands D-E.
Parsing:  F-G(ish)

Define commands F-G(ish); still more G options to go.
Parsing: Finish documenting existing commands

This tentatively makes a nod to every command's syntax, albeit in some cases
with some obvious hand-waiving over some of the more finer details.  If
they're needed, these can be written up as we go.
Cleaned up ebnf and converted to abnf (rfc5243).
Abnf corrections.
Abnf work (ADDBUTTONSTYLE to KEEPRC).
Abnf work (KEY to POPUP).
Abnf work.
Details out command syntax.
Definitions of STRING and TOKEN.
Fix Abnf character ranges.
Correct spelling:  BOOLOR_TOGGLE -> BOOL_OR_TOGGLE
Parsing:  Few cleanups

* Make note of a few definitions which will need defining;
* Flesh out AddButtonStyle a bit more.
Parsing:  Some restructuring

* Collate obsolete / removed commands together;
* Flesh out some common definitions (DECORNAME, etc)
Remove/reindent stray comments
Fix MENUCONTEXT definitions

The ABNF syntax wasn't quite right.
Abnf work