Change media gallery to be media type agnostic

[DieterHolvoet]

Summary

Change the media gallery to work with any media type instead of just image.

Basic example

• Add a type filter
• Use MediaSourceInterface::getMetadataAttributes for the metadata displayed in the preview
• Use the media entity view display for previews

Motivation

To be able to use the gallery for more than just this one image media type, e.g. local videos