#844: rose edit: fix jumping page bug

This fixes a problem where the page tree will jump to an incorrect row after page launch. This can only really be seen for a large configuration with many tabs open. Launching many tabs, then selecting one of the middle tabs and then left-click launching pages should produce jitter with the old code.

The left-click launching destroys the current page and makes a new one in the tab. This was registered as two separate page changes, necessitating two page tree jumps, which caused the problem - it should just be one.

This closes #844.

@arjclark, please review.