- page grouping proxy model:
  + more checks when an index is requested
  + be sure to return really unique indexes
  + proper calculation of the column count
  + directly reset() (instead of just a layout change) when we rebuild the structure
- author grouping proxy model:
  + more checks against invalid indexes
  + behave nicely on empty tree

svn path=/trunk/KDE/kdegraphics/okular/; revision=818419