Add plugin for taking photos on remote device

Summary: This plugin sends a request for a photo to the connected device. The device then sends the photo back, currently via the SharePlugin.

Test Plan:
kdeconnect-cli --photo
Check for photo in Downlaods folder

Reviewers: KDE Connect, broulik, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: sredman, albertvaka, apol, ngraham, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D18140