Groups books by author when sorting by author

Closes #2
Re-layout book if window size changes
Side-by-side lists for author sort with wide screens

There was some trickery with the Page flickable to get both lists to
line up below the header.  That works, but you can see the author list
through the header when the list of that author's books are shown.
Animate between lists in author sort mode when narrow

Now, the two lists are always side-by-side.  But in the not-wide case,
each of their widths takes up the whole screen, and we move them left
and right to show the correct one.  We need to redo these calculations
each time the width is changed.