Make Esc close the map editor config dialog.
Fix the map preview for archives with only one map.

Building the map list would try to restore the selection to whatever it
was originally, defaulting to -1.  But selecting item -1 actually means
to select every item, which for a single-selection listbox means
selecting the last item, which is in the case of a single map also the
first item.  This happened before the onMapSelected handler was bound,
and because the first item was already selected, the constructor's
attempt to select the first item would be ignored, and the map preview
would never be drawn.
Focus the map canvas by default.
Fix removal of zero-length lines when merging vertices.

Code was checking the global list of lines rather than the vertex's
connected lines, so it was almost never triggering.
Add missing question mark to map editor save prompt.
Force re-rendering the map canvas on property changes.

This ensures that lines correctly change color when they gain or lose a
special.
Set focus when the props dialog opens, and refresh the window on close.

Now the in-window properties panel updates to show any changes made with
the dialog.
Make MapLine->needsTexture aware of ZDoom's Plane_Align.

This is a pretty simple slope case to check, and it gets rid of a lot of
map check noise.
Remove games/include/udmfprops_zdoom.cfg.

ZDoom isn't a game, nothing includes this file, and it's identical to
ports/include/udmf/props_zdoom.cfg.
Remove games/include/udmfprops_boom.cfg.

Same story: not actually a game, unused, and duplicate of another file.
Remove games_old.

These files conflate games with ports, don't seem to be loaded by the
app anywhere, and haven't been touched in a year and a half so are
surely way out of date anyway.
Remove a whole bunch of unused config files.

Turns out all of these were renamed ages ago, but the originals have
lingered.
Merge pull request #122 from eevee/misc-minor-fixes

Fixes for a few minor miscellaneous annoyances.
Merge pull request #123 from eevee/game-config-cleanup

Delete unused game config files.
Removed the int functions from OPL emulator.

Instead, the conversion to SFML's only accepted format is handled by the
OPL player.
Merge branch 'master' of https://github.com/sirjuddington/SLADE into Music