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.

modules/mono/csharp_script.h

@@ -326,17 +326,13 @@ public:
 };
 
 struct CSharpScriptBinding {
-	bool inited;
+	bool inited = false;
 	StringName type_name;
-	GDMonoClass *wrapper_class;
+	GDMonoClass *wrapper_class = nullptr;
 	MonoGCHandleData gchandle;
-	Object *owner;
+	Object *owner = nullptr;
 
-	CSharpScriptBinding() :
-			inited(false),
-			wrapper_class(nullptr),
-			owner(nullptr) {
-	}
+	CSharpScriptBinding() {}
 };
 
 class ManagedCallableMiddleman : public Object {