Decouple Overlay from web content's fullscreen state

hidden: true means that Overlay.qml is animated to offscreen. Could also be renamed to have or moved inside.

Other approach could be that this hidden property is removed and line 69 does ```overlay.visible = false``` and 240 set overlay to visible like ```overlay.visible = animator.state !== "fullscreenWebPage"```

Enabled of Overlay.qml would then be enabled: visible. Would add comment to the Overlay.qml @ 89 and Overlay.qml @114 (Browser.OverlayAnimator) that visibility is changed by OverlayAnimator. But this is just a suggestion.