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/script_language.cpp

@@ -34,6 +34,7 @@
 #include "core/debugger/engine_debugger.h"
 #include "core/debugger/script_debugger.h"
 #include "core/project_settings.h"
+
 #include <stdint.h>
 
 ScriptLanguage *ScriptServer::_languages[MAX_LANGUAGES];