Merged sdlcurse.cpp and sdltiles.cpp

(See Issue #3135)
These two files were mostly identical, and prone to consistency errors as a result. I merged them into a single file (sdltiles.cpp) and removed almost all the redundancy. The source compiles the same way, only now SDLTILES should now be defined alongside TILES to compile the tileset version (previously they were mutually exclusive).

Remaining issues:
In cursesport.cpp, there's a conditional compile flag at the top that only applies on non-windows builds:
```defined TILES ^ defined SDLTILES```
I have no idea what this exclusive OR is supposed to do, since TILES and SDLTILES previously caused redefinition errors when used together anyway. It seems to be useless now, but I'm running on windows so I can't tell for sure. If anybody could explain this I'd appreciate it.