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.
If the main window was maximized before, maximize it when we
restart.
Changed the preferences to record the width of the right panels,
rather than the splitter distance, which is actually the width of
the middle of the screen.
Seems to work correctly on my non-uniform multi-monitor setup. I
added a check to confirm that the middle of the title bar falls in
the working area of the screen that WinForms thinks we're in, so it
shouldn't be possible to "lose" a window off the edge by dragging
it or by changing screen resolutions.
