Preserve popup look on page zooming

[dvdvdmt]

The popup tries to look the same independent of a page zoom level. Unfortunately there is no way of getting zoom level reliably (https://stackoverflow.com/questions/1713771/how-to-detect-page-zoom-level-in-all-modern-browsers).

• Using outerWidth/innerWidth will give wrong results if a side bar is open (eg. dev tools, menu in FF).
• The usage of 'vw' unit has the same flaws as outerWidth/innerWidth approach.
• The window.devicePixelRatio (DPR) changes on zoom but you can't rely on it on high DPI devices because there is no way of getting base DPR that corresponds to zoom 100% (https://www.w3.org/community/respimg/2013/04/06/devicenormalpixelratio-proposal-for-zoom-independent-devicepixelratio-for-hd-retina-games/).

Currently extension specific API browser.tabs.getZoom() is used to get tab zoom factor. Despite of knowing zoom factor the minimal font size on large zoom levels can't be rewritten.