Player shortcut improvements

[zubeydecivelek]

These keyboard shortcuts are implemented after user feedback:

• Spacebar for play/pause
• Left/Right Arrow keys for seeking (+/- 5 seconds)
• F key for fullscreen
• Up/Down Arrow keys for volume control
• C key for subtitles
• M key for mute/unmute

See the commit: 27aac94

Possible improvements:

• play/pause icon appears when using the shortcut or clicking on the video screen
• volume up/down/mute icon with value when shortcut used
• seek forward/back icon with positive or negative value when shortcut used
• subtitles/cc icon with value (e.g. "English", "French", etc...) when shortcut used
• the fullscreen action does not produce visual feedback
• in no case the actions made trough the toolbar triggers a visual feedback
• a new action cancels the animation of the current one, if present (e.g. spamming or keeping pressed the volume button)