Add multimedia sub-categories

Summary: FEATURE: 388933

Test Plan:
Tested in KDE Neon. The top 10-15 entries in each category are good, but the {nav Audio Players} and {nav Video Players} categories have far too many items as a result of poor app category tagging. Further work is needed to reduce these lists to manageable levels, but hopefully this is better than nothing for now.

Audio and Video editors:
{F5716821}

Audio Players:
{F5716822}

CD and DVD:
{F5716823}

Video Players:
{F5716830}

Reviewers: #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10622