[IMP] web_tour: tip location

Choosing where to place a tip is a difficult problem.

If placed in the body element, the tip have to be repositionned on each
scroll event which is performance-consuming and induces bugs (firefox
scroll event, tips which is over an element while the anchor is under,
...).
If placed in the anchor parent which scrolls, all the above problems
are solved but when the tip opens it cannot overflow its parent
boundaries, which is not possible in many contexts (modals, ...).

The solution here is to change the tip location when it opens/closes.
When closed, it is placed in the anchor parent which scrolls. This
allows to not have to reposition the tip on each scroll. The tip is
also naturally hidden when its anchor is not visible on the screen.
When opened, the tip is placed in the body, where it can be placed
on the top of anything in the DOM.

The transition from one state to another is not noticable as the
position does not have to be computed when the tip is moved in the
DOM and the animation is purely CSS.

Note: however, this commit does not solve the firefox bug of closing
the tip when the window scrolls. Here, the temporary solution is to
let the tip be wrongly position if the user scrolls when a tip is
opened (this only occurs when using the mouse wheel anyway).

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