Performance improvements for 0.65.2

[Frank Niessink]

Hi all,

I've checked in a number of performance improvements:
- Don't update the dynamic menu's (the View menu's that depend on the
active viewer) in idle time but only after the active viewer is
changed.
- Use caching for root items in the task sorter.
- Use caching for indices in the tree viewer.

These changes were checked in on the Release0_65_Branch.

Hopefully this is a significant step towards better performance. I'd
appreciate any feedback.

Cheers, Frank