runDialog: Use ClutterText::activate for enter handling

Instead of consuming the event in front of the input method. Enter is sometimes overriden by those, so it seems better to let the IM handle the key event, and react later to it if it got propagated anyway. That is what ::activate does, so use this signal.

This used to work before ClutterInputMethod/InputFocus because the IM received the events directly from stage captured events. This is not the case anymore.

Closes: #440 (closed)