fixes #873

fixes #857

- This ensures that gstreamer bus operations work
on macOS and windows, such as enumerating devices.

Allows you to zoom and pan now.

relates to #647

fixes #439
Workaround for https://bugreports.qt.io/browse/QTBUG-97401
fixes #810

This makes them less annoying when scrolling or accessing the popup
menu.

See also #860

- Add scrolling
- Only display ~7 items at once

Not only should this prevent hickups, it also fixes a potential crash.

- If an inline image has specified a height, add parameters to the
  image:// URI.
- Add scaled to the parameters, the images would be cropped otherwise.
- Extract the height from image:// URI and use it for requestSize.
- Use scaledToHeight instead of scaled.