Added a plugin to present slideshows
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