Positioning: ngAfterViewChecked re-positioning not foolproof

[pkozlowski-opensource]

It seems like the ngAfterViewChecked is not always called in the order we would expect. This can lead to badly positioned elements. To reproduce go to a demo page for popovers and:

• open a popover
• toggle source code view for the same demo for which you've opened a popover

Please note that the problem doesn't happen if you open other demo. So it only affects elements under the same root.

[pkozlowski-opensource]

Other issue: the current positioning method doesn't take resize events into account.

[pkozlowski-opensource]

#709 tackles tooltips and can serve as an example for other directives. Please note that we should merge #708 first to avoid tricky merges.