[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
if we're gonna use a constant (`defaults.VIEWPORT`), why not pre-calculate the QSize object and call `self.web_page.setPreferredConentsSize` here directly?