Make it possible to stream only input

Summary: No need to share our screen contents to be able to move the mouse.

Test Plan:
Ported KDE Connect to it, works.
https://invent.kde.org/kde/kdeconnect-kde/merge_requests/199

Reviewers: #plasma, #kde_connect

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26875