Additional keyboard shortcut improvements

Here are some useful keyboard shortcuts pulled from VLC, SMPlayer or Celluloid.

Playlist

• Ctrl + Shift + O: Add file to playlist
• Ctrl + Shift + L: Add location to playlist (#84)
• Ctrl + Shift + S: Save playlist
• Ctrl + Shift + P: Show playlist
• N: Next
• P: Previous

Audio

• M: Mute (alternative to existing 0)
• K: Next Audio Track
• Up: Volume Up
• Down: Volume Down

Subtitle

• J: Next Subtitle
• V: Toggle subtitle

Others

• Ctrl + P: Tools > Preferences
• Ctrl + H: Toggle Controls Bar (#85)
• Left or Right: Seek back or forward 5 seconds (#87)
• Alt + Left or Right: Seek back or forward 10 seconds
• Ctrl + Left or Right: Seek back or forward 1 minute
• Ctrl + Alt + Left or Right: Seek back or forward 10 minutes