NeulandTox: Fix idle looping

The idle function on_typing_change_idle() was called endlessly because
it didn't return FALSE. Also use G_SOURCE_REMOVE instead of FALSE in all
idle functions now to be clearer.
Don't allow sending to offline contacts

Ignore pressing return in the text entry when the contact is offline,
otherwise the message will go nowhere. Also, show an info bar, informing
the user that they currently can't send messages because the contact is
offline.