Improved video

This merge request makes video recording work again by not enforcing a fixed frame rate. It also includes the display of the elapsed recording time, video thumbnails, as well as some smaller improvements.

Before I consider this merge request ready I plan to extend the PreviewArea to work for videos too. I tried this some time ago and did not manage to get a thumbnail shown. If this does not work again the backup plan would be to just draw some playback or video icon in the preview (that then links to the actual recorded video), if that is acceptable.