Switch view to "scan" when selecting 2-page-view from the panel

[ipf]

When touching/clicking the 2-page icon in the header-bar, the view has to switch to the scan, to be consistent with the other page-selecting actions

[t11r]

Maybe we should not automatically switch the view at all. Changing the page does not only change the image, but also the contents of the Fulltext and Export panels, and we cannot predict the user’s intention.