fix about button icon and loading of the parserBackendModel

Adds an icon for the about dialog. I waited until now for that in the hope that something useful would appear in ubuntu-mobile-icons. But there hasn't so I wrote a "i" in Ubuntu font (italic, bold) and colored it like the other icons in ubuntu touch.

Also, it fixes a bug that slipped in when switching to the models: The parserBackendModel wasn't loaded any more.