Make sure project manager gets editor hint set

Previous fix in e8e06b2 worked in most cases but not if you run e.g.
'godot -', where the '-' argument would mean that 'project_manager'
is false and yet that's what will be opened eventually.

(cherry picked from commits e8e06b2c9a
and c0df3b147e)

main/main.cpp

@@ -1703,6 +1703,7 @@ bool Main::start() {
 #ifdef TOOLS_ENABLED
 		if (project_manager || (script == "" && test == "" && game_path == "" && !editor)) {
 
+			Engine::get_singleton()->set_editor_hint(true);
 			ProjectManager *pmanager = memnew(ProjectManager);
 			ProgressDialog *progress_dialog = memnew(ProgressDialog);
 			pmanager->add_child(progress_dialog);