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.
Every once in a while, SourceGen will become unresponsive when it
tries to show a MessageBox. In the debugger you can see the GC
running frantically, but the stack trace is just sitting on the
MessageBox show call. Apparently, if you don't specify a parent
window argument, the MessageBox will occasionally end up behind
everything else, and you can get stuck.
I'm not sure what the GC frenzy is about, or whether this will fix
what I'm seeing, but it's easy to do and might solve the problem.
cf. https://stackoverflow.com/q/3467403/294248
