Summary:
This adds a plugin with buttons for next and previous slide and fullscreen.
It also allows you to use volume up/down to change slide, even when the screen is off.
It can be extended to show do more presentation-related stuff eventually.
On the desktop side, it uses the already present 'mousepad' (keyboard input) plugin.

Test Plan: Tested manually.

Reviewers: #kde_connect, nicolasfella, apol

Reviewed By: #kde_connect, nicolasfella, apol

Subscribers: apol, nicolasfella, kdeconnect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D14430