Optimize backdrop rendering

[VladimirAnaniev]

Noticed that the backdrop was removed and then re-drawn on each step of the tour.
These operations for removing and then creating new DOM elements are expensive and can be avoided as only a couple of styles are changed. We can only update the needed properties when needed.
Other benefit of reusing the same elements is the ability to incorporate animations on transition later.

I have implemented this optimization with this change.

[tnicola]

Hello @VladimirAnaniev , Thanks for your PR. Can you please resolve the conflicts?