'dual' view: thumbnails should correspond to main document

[ijk]

Binary package hint: evince

It should be visually obvious which 2 thumbnails are currently displayed, a box or other visual element should enclose the pair of currently shown pages.

When the thumbnail sidebar is wide enough for multiple pages to be shown horizontally:
* The pages should be shown with less spacing between pairs of pages that will be on screen at the same time.
* The first available thumbnail space should be skipped to correspond with the first page being shown by itself.

When dual mode is first chosen the thumbnails should change to 2 columns (ie 1 column of pairs) provided it doesn't change the zoom of the current document (most likely in 'best fit' mode).