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_buffered.cpp

@@ -166,11 +166,3 @@ Error FileAccessBuffered::get_error() const {
 
 	return last_error;
 }
-
-FileAccessBuffered::FileAccessBuffered() {
-
-	cache_size = DEFAULT_CACHE_SIZE;
-}
-
-FileAccessBuffered::~FileAccessBuffered() {
-}