Adds mute and direct sound settings keybindings

This patch adds a mute key ('*') so sound can me muted/unmuted.
It also provides keys from 'A' to 'M' that sets the sound level directly to a specific level between 0% and 120% (e.g. 'D' sets sounds to 30% volume, 'F' key to 50%, etc...). The formula used to convert between amplitude and dB maps 0% to -60dB (VOLUME_MINIMUM) and 100% to 0dB.

Those new keys should help omxplayer scripters around the world managing volume more efficiently.