macosx: Only set placeholder image and fetch artwork when necessary, preventing constant flashing of placeholder image

Currently, every time the control bar is updated (say to update the progress bar), the artwork flashes:

Screen_Recording_2022-06-18_at_19.59.36

This is because on each update, the artwork is set, even if it hasn't changed, with the placeholder image being set before the artwork has been fetched. Thus the flashing.

This MR changes this behaviour. VLCImageView stores a copy of the most recently set artwork URL and checks first if a URL has set, and if this URL is the same as the one about to be set, before applying the placeholder image and the new artwork URL.

After fixes:

Screen_Recording_2022-06-18_at_19.59.36