Expose ClueList#getTitle to Lua and use where appropriate

[jpd236]

ClueList#getTitle is currently not exposed to Lua.

My interpretation of this field is that it is meant to be a display title for a clue list, whereas the string key in Clues is more of a semantically-meaningful direction. Today both of these are always set to the same value, but ipuz supports distinct values for these two fields (see #219), so it'd be nice to be more consistent about using the right field in the right places.

I think I can handle the C++ callers, but there's at least one in Lua that should be updated as well, and perhaps others:

https://github.com/mrichards42/xword/blob/master/scripts/xworddebug/html.lua#L91

This prints the direction, but would probably be better printing the display title instead.