Video encoding support

[rocksdanister]

When video file is imported, provide additional options:

• Convert from one codec to another (x264 is optimal for performance.)
• Resolution change (1080p is optimal for performance.)
• Trim/cut start and end-time.
• Wallpaper performance presets (resolution, bitrate etc..)
• Remove audio track.

The UI should be similar to Depth Wallpaper dialog with a smooth preview and based on the file should provide suggestions for optimal performance.

Should be possible directly using mpv:

https://github.com/mpv-player/mpv.io/blob/master/source/guides/encoding.rst

https://mpv.io/manual/stable/#encoding-o

[rocksdanister]

If user selects x264/unsupported video file for webpage wallpaper transcode to vp9/vp8 by prompting user: https://github.com/rocksdanister/lively/wiki/Web-Guide-VI-:-Webpage-Video-Support#converting-existing-videos-to-webm