Set proper component name for plasma-discover

Summary:
Plasma discover installs desktop file as org.kde.discover, but it
instead sets its component name as muondiscover, and breaks the RDN
scheme.

this fixes window icon for discover

Reviewers: Plasma, graesslin, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2587