Support sending messages with numpad enter

Currently only the "main" enter key leads to messages being sent.

Currently:

• https://code.briarproject.org/briar/briar-gtk/-/blob/a46245005fdd2d65bb05b6766325d025dd70544a/briar-gtk/briar_gtk/private_chat/private_chat_presenter.py#L193
• https://code.briarproject.org/briar/briar-gtk/-/blob/a46245005fdd2d65bb05b6766325d025dd70544a/briar-gtk/briar_gtk/private_chat/private_chat_presenter.py#L273

Formerly: https://code.briarproject.org/briar/briar-gtk/-/blob/0.1.0-beta2/briar-gtk/briar_gtk/containers/main_window.py#L187