Improve UX on iphone Safari

[ClementSparrow]

• directional gestures are sometimes interpreted as flick-to-scroll by Safari, especially in landscape mode (probably due to starting the gesture outside the canvas, in the area at the bottom of the screen that is used to trigger the display of the app's icons?)
• the button bar is sometimes hard to open (small button)
• unnecessary margins on small screens?
• page title can be partially hidden by the game canvas.
• fullscreen button on the bottom right not working?

See also https://github.com/increpare/PuzzleScript/issues/677, although I don't have the double-tap zooming issue on my iPhone, and I think it's important to be able to zoom on buttons on small screens. So I will not merge the commit that solves that issue for the moment.

[ClementSparrow]

Also, it seems the mute button is not shown.