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/translation_loader_po.cpp

@@ -212,6 +212,3 @@ String TranslationLoaderPO::get_resource_type(const String &p_path) const {
 		return "Translation";
 	return "";
 }
-
-TranslationLoaderPO::TranslationLoaderPO() {
-}