[trunk] R4754 Navigator issues

1. Create a new score of several hundred measures, so several pages are created.
2. Select a measure on the first page, and shift-select a measure on the last page.
3. Press Ctrl-Delete to remove the selected measures.

The blue box in the navigator window can now be moved left (going backwards) farther than the first page.

Also, the navigator blue box should be moved back so that it focuses on a page, rather than nothing.