imwayland: Suppport terminal purpose
The zwp_text_input_v3 Wayland protocol specifies a text purpose under the name of "terminal". As the Librem 5 is in the hands of customers, the need to implement this part of protocols has been strongly indicated in order to have a useable on-screen keyboard for terminals.
The purpose can be tested using the attached script under a phoc (or wlroots) wayland session.