support for defining autostart apps in lnf packages

Summary:
meant to support layouts that want external apps such as lattedock
or conky, it adds services (in the defaults file they're saved as
the filename of the desktop file) in autostart and starts immediately
the app as well.
before applying a lnf, it removes from autostart anything the old
package had and attempts to stop the apps

Test Plan:
tried to start/stop lattedock with the mechanism
some fallback may be needed for non kde apps

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: ngraham, mvourlakos, apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9288