WIP: [preview] Add preview widget
This enables the preview plugin to generate preview widgets for all mime types.
- Previews for images are generated like before
- Non-previewable files are displayed using their respective mime type icons
- Locations provided by geo-URIs will additionally display specific location infos
For the 'most used' actions, two buttons were added for easy accessibility.
- if previews are enabled for all urls, a widget containing a link icon will be generated