switcherPopup: Use roundtrip time when the popup is modifier-less

The noModsTimeout obviously finishes inside a timeout callback, which means global.get_current_time() will return Clutter.CURRENT_TIME (ie. 0) when called inside it because it's not called while handling an event. This means when switching apps or activating a window, the timestamp passed to activate_window will be 0, which is the reason why the altTab switcher is currently broken when using modifier-less keybindings.

Fix that by using meta_display_get_current_time_roundtrip, which always return a valid timestamp, instead of shell_global_get_current_time.