Explicit box model definition for box model dependent sizing

The zoom in/out tooltips are clipped when a css reset sets the
box-sizing to anything other than content-box. This breaks the
sizing of these tooltips, because they rely on content-box
sizing. This issue can e.g. be seen when using a bootstrap 3.x
bootstrap.css
Merge pull request #1771 from ahocevar/tooltip-box

Fix zoom in/out tooltip sizing for box-sizing css resets