Not complete yet, but has @KEY and @DSP, enough to get prompt.

I'm about to reuse sdc.c in sms.c and the old directory structure
was becoming awkward.

I'm planning on adding other subfolders. 8086 for example...

I thought it wasn't possible with XCB to transform keycodes into
symbols for the current keyboard mapping, but I hadn't looked
hard enough.

Gave more priority to CPU in the emulator because initialization would
otherwise be horrendously slow.

Previously, it would never get out of the event loop.

It seems to be picky about where it accepts its link flags.

I implement the screen using XCB which is much more friendly
than z80e's SDL+CMake for development machines that want to install
minimal dependencies (for example, a port-less OpenBSD rig).