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.h

@@ -43,7 +43,7 @@ public:
 	virtual bool handles_type(const String &p_type) const;
 	virtual String get_resource_type(const String &p_path) const;
 
-	TranslationLoaderPO();
+	TranslationLoaderPO() {}
 };
 
 #endif // TRANSLATION_LOADER_PO_H