Show message on iOS to request user to enable devicemotion manually

Since iOS 12.2 Safari devicemotion API comes disabled by default. The user has to enable manually on settings. We could detect iOS and if the API is disabled we can display a message above the enter VR button asking the user to enable.

[dmarcos]

Can just one feature detect devicemotion? A bit ugly but maybe:

var deviceMotionEvt;
setTimeout(function () { 
  if (!deviceMotionEvt && AFRAME.utils.isIOS()) { showMessage(); };
}, 1000);

window.addEventListener('devicemotion', function(event) {
  deviceMotionEvt = event;
});

[dmarcos]

Example on how 8th wall deals with the issue: https://github.com/aframevr/aframe/issues/3976#issuecomment-493035158