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.

editor/editor_plugin.cpp

@@ -913,16 +913,6 @@ void EditorPlugin::_bind_methods() {
 	BIND_ENUM_CONSTANT(DOCK_SLOT_MAX);
 }
 
-EditorPlugin::EditorPlugin() :
-		undo_redo(nullptr),
-		input_event_forwarding_always_enabled(false),
-		force_draw_over_forwarding_enabled(false),
-		last_main_screen_name("") {
-}
-
-EditorPlugin::~EditorPlugin() {
-}
-
 EditorPluginCreateFunc EditorPlugins::creation_funcs[MAX_CREATE_FUNCS];
 
 int EditorPlugins::creation_func_count = 0;