Fixed a display glitch caused by tooltips

The glitch is really easy to reproduce.

1. Open the demo: http://demo.onokumus.com/metis/index.html
2. Move cursor over the top right icons in the "Metis Top Navigation Bar".
3. Notice that the buttons move horizontally by 1-2px when the tooltip appears.

The fixed: Append the tooltip element as a last child to the button's parent container instead of inserting as the next sibling.