Fixed various Clang++ warnings

With these patches, I can use clang++ to build OpenXcom with quite a few warning flags set:

    -Werror -Wall -Wextra -pedantic -Wno-c++11-long-long

The `long long` warning (`long long` is an extension before C++11) occurs in `Engine/FixedFloat.h`, where the scratch space must be twice the size (`long long`) of an actual float (`long`) when multiplying or dividing. I'm not sure if it's worth the effort to turn this into pre-2011 standard C++.

Please note that 80c03a0 removes a bunch of unused members. A few of these changes are quite suspect, and I suggest that that patch is actually double-checked to see whether it points to missing business logic.