BaguetteBox must not exit fullscreen if it did not enter fullscreen itself.

[LeoniePhiline]

The closeButtonClickHandler calls exitFullScreen() even if the document was in fullscreen mode before baguette box was opened, and even if options.fullScreen is false.

This breaks websites & apps handling their own fullscreen mode:
When the website/app is in fullscreen mode, opening and then closing a baguetteBox will make the entire app exit fullscreen. This is breaking UX badly.

Options:

1. At least check options.fullScreen (like before calling enterFullScreen()) and do not exitFullScreen() if options.fullScreen is false.

2. Track the fullScreen state in showOverlay() and only exit fullScreen if the document was not already in fullScreen mode during showOverlay().

3. Ideally replace document.fullscreen.

• It is deprecated and hardly supported any longer: https://developer.mozilla.org/en-US/docs/Web/API/Document/fullscreen
• Instead, use the current fullscreen API.
  • You can apply fullScreen to the baguetteBox root element instead of the entire document.
  • If document.fullscreenElement is already not null, then do not go fullscreen at all.
  • Also, do not exit fullscreen if document.fullscreenElement is not the baguetteBox element.