net-im/signal-desktop: Add new port

Signal is a cross-platform centralized encrypted messaging service developed
by the Signal Technology Foundation and Signal Messenger LLC.
It uses the Internet to send one-to-one and group messages, which can include
files, voice notes, images and videos. It can also be used to make one-to-one
and group voice and video calls,[16][17] and the Android version can optionally
function as an SMS app

WWW: https://signal.org/

Special thanks to: tagattie@ for maintaining the electron ports