Proposed changes for issue #162: Request for custom video cover art

Right-click on video now has a 'change cover' option that opens a file picker to select the image file. Tested on W10 and Xbox

Signed-off-by: Martin Finkel martin@videolabs.io

Work by Evan Blackstone.

• Test on mobile
• Add translations
• Can we trigger an image refresh without rescanning the whole library?
• What if the user selects a huge picture? Is the buffer cleared properly in the helper functions?