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.
Merge pull request #122 from eevee/misc-minor-fixes

Fixes for a few minor miscellaneous annoyances.