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/io/file_access_memory.cpp

@@ -193,8 +193,3 @@ void FileAccessMemory::store_buffer(const uint8_t *p_src, int p_length) {
 	copymem(&data[pos], p_src, write);
 	pos += p_length;
 }
-
-FileAccessMemory::FileAccessMemory() {
-
-	data = nullptr;
-}