reduce the number of calls to updateText on a DiveTextItem

[Henrik Brautaset Aronsen]

Tomaz Canabrava wrote:
> This should bring a huge amount of speed for mouse movements.
> here it goes from 17% CPU to 9% CPU moving the mouse frenetically.

Much much better now.  A couple of weeks ago I was able to get 70-80% 
CPU usage by moving the mouse around in the profile.  Now I can't get it 
higher than 20%.  Great work!

Henrik