chat widget: We don't need to name all text marks

We store pointers to these two already, so no names are needed.
Don't lose '<contact> is typing ...' info

When the typing info is visible the moment we send a message, make sure
to insert it again below our message.
Remove neuland_chat_widget_get_show_is_typing()

"is-typing" is a property of NeulandContact after all, so use that
instead.
Implement setting our typing status