Document checker dialog: fix selection, allow multiple selection, limit color...

Document checker dialog: fix selection, allow multiple selection, limit color background and striked out text to a specific column Selection was broken because abstracttreemodel disabled selecting depth 1 items, so I reimplemented the flags method in documentcheckertreemodel to fix it. I also limited the red background to a specific column since having an all red background was a bit ugly, same for striked out text that is now limited to the url.