Added explicit storage for MapData constants.

Uncovered by compiling with clang while -fno-rtti is set, in which case
linking fails.
Removed unused members (and respective constructor parameters)

These members were actually unused, but please double-check whether that is
intended.
De-virtual-ized the load(const YAML::Node&) methods

The use of virtual is incorrect for load, since they're not really overloaded.
The signatures of load(...) for quite a few classes take extra parameters,
which indicates it's more like a constructor than anything else.
Made unsafe conversion between object pointers and function pointers explicit
Suppressed miscellaneous Clang++ warnings

* File should end with an End Of Line character
* No semicolons where they don't belong
* Signed/unsigned arithmetic
* A struct that is a friend is still a `friend struct`
* Use an unused parameter (no other way, due to preprocessor stuff)
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of https://github.com/rhymoid/OpenXcom

Conflicts:
	src/Battlescape/ActionMenuItem.cpp
	src/Engine/ShaderMove.h
Code cleanup.