WIP: Add more command line options

[Rafostar]

• [x] allow setting GStreamer elements (audio/video filters and sinks)
• [x] set volume
• [x] set speed
• [x] enqueue media to primary app instance
• [x] create new application window
• [ ] set queue progression mode
• [ ] start fullscreened
• [ ] fix remaining options description in --help

Closes #294 Closes #112 Partially implements #76