virtual-keyboard: Add new virtual keyboard protocol

Provides the ability to emulate keyboards by applications. Complementary to input-method protocol.

The interface is a mirror copy of wl_keyboard, with removed serials, and added seat binding.

I've sent this patch to the mailing list before, but now the mailing list is unused, so I'd like to give it more visibility.

The protocol is successfully being used as the primary input method on the Librem 5 phone.