Rather than have each assembler get its own app config string for
the cross-assembler executable, we now have a collection of per-
assembler config items, of which the executable path name is one
member. The Asm Config tab has an auto-generated pop-up to select
the assembler.
The per-assembler settings block is serialized with the rather
unpleasant JSON-in-JSON approach, but nobody should have to look
at it.
This also adds assembler-specific column widths to the settings
dialog, though they aren't actually used yet.
