[WIP] Set default viewport size to layout content

Experimenting with http://qt-project.org/doc/qt-4.8/qwebpage.html#preferredContentsSize-prop

Also using a wider default viewport (1366x768): this can be reverted of course
Actually, I was thinking this `setPreferredContentsSize()` thing could also be used to set the rendering width for different targets, probably with a new `width` parameter, to be combined with `viewpwort=full`

* desktop: width=1366px
* smartphone: width=320px
* tablet: width=768px