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/thread_work_pool.cpp

@@ -29,6 +29,7 @@
 /*************************************************************************/
 
 #include "thread_work_pool.h"
+
 #include "core/os/os.h"
 
 void ThreadWorkPool::_thread_function(ThreadData *p_thread) {