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/os/main_loop.cpp

@@ -60,12 +60,6 @@ void MainLoop::set_init_script(const Ref<Script> &p_init_script) {
 	init_script = p_init_script;
 }
 
-MainLoop::MainLoop() {
-}
-
-MainLoop::~MainLoop() {
-}
-
 void MainLoop::init() {
 
 	if (init_script.is_valid())