New plugin: Find this device

Summary:
Allows other devices to make this device discoverable via a
kdeconnect.findmyphone.request command, if running.

Currently supports playing a sound.

Counterpart to FindMyPhone plugin

Test Plan:
Connect with other device running KDE Connect (with Plasmoid).
Select a working play sound in the Find My Device plugin settings.
On other device trigger Find My Phone button for this device in KDE Connect
Plasmoid and notice this device playing the configured sound.

Reviewers: KDE Connect, nicolasfella

Reviewed By: KDE Connect, nicolasfella

Subscribers: kdeconnect, sredman, mtijink, apol, nicolasfella

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D11773