New Matrix IM desktop client and support-library.

Quaternion is a Matrix IM desktop client; Matrix is a "modern" IM
solution, based on federated messaging and optional end-to-end
encryption. The protocol and most implementations are entirely
Open Source.

This commit introduces a support library, net-im/libqmatrixclient,
and one user of that library, net-im/quaternion. The library itself
is used by other Qt-based Matrix IM clients as well but I needed
*one* client, not three of them.

The pkg-descrs are awful short, but there's really not much more
to say than "a desktop Matrix IM client", since that's what it is.
Listing features seems like useless fluff, since those are inherent
in Matrix protocol support (avatars, image previews, long messages,
etc.).