Fix tooltip positioning with regards to viewports

The getPosition function was simplified with a new function,
getViewportBounds, was added to specifically
tackle the cases where we’re looking for the viewport bounds.

‘absolute’ and ‘fixed’ tooltips are now matched against the screen
where other ones are made to fit inside the viewport element.
You only use this variable once, could just as well inline it. Also, `this.$element` is already a jQuery object so no need to wrap it in `$()` again.