Rename cmakelists.txt to CMakeLists.txt

CMakeLists.txt is the proper name, and the only one
that's accepted by CMake on case-sensitive filesystems.
src/GameFloor.h: Fix non-matching case in an include

This way, the compiler can find the file on case-sensitive filesystems.
Merge pull request #1 from crumblingstatue/case-sensitivity

Fix case-sensitivity issues