WIP: Resolve "Demo Screensaver"

Please make sure your MR is ready to be merged !

• You added the changes in CHANGELOG.md
• You described the MR as below
• Press a button make the demo mode off when running game
• Press a button make the demo mode off when browsing in ES
• Do not considere an empty directory
• System choice in options
• Independant module for es-prime

Fixes #32 (closed)

Changes :

• Added demo mode for screensaver