Mp4-Mux-Tool v1.21

Updates

• Updated included mp4box to v2.3-DEV-rev724-g8684dfbcc-master
• Updated included mkvextract to v82.0
• Updated pymediainfo to v6.1.0

Changes

• Removed the FPS entry box (this is not needed, mp4box does not need fps supplied to it, this was just for visual purposes and caused confusion)
• Did some organization to the UI to make the entry box's, buttons have nearly the same geometry and look a lot better through out (only minimal changes, this app is still in maintenance mode until the new version comes out)