Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

core/math/random_number_generator.h

@@ -65,7 +65,7 @@ public:
 			return ret % (to - from + 1) + from;
 	}
 
-	RandomNumberGenerator();
+	RandomNumberGenerator() {}
 };
 
 #endif // RANDOM_NUMBER_GENERATOR_H