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/gdnative/pluginscript/pluginscript_script.cpp

@@ -550,11 +550,6 @@ MultiplayerAPI::RPCMode PluginScript::get_rset_mode(const StringName &p_variable
 }
 
 PluginScript::PluginScript() :
-		_data(nullptr),
-		_desc(nullptr),
-		_language(nullptr),
-		_tool(false),
-		_valid(false),
 		_script_list(this) {
 }