SciDAVis - Improvements SVN-temp

- inverted the behavior of the validity attribute:
now valid is the default for any row and you use
setInvalid()/isInvalid(). Since rows are rarely
invalid, this seemed the better choice.
- removed validityChanged()/validityAboutToChange()
from AbstractDataSource.
Use dataChanged()/dataAboutToChange() instead.
- some small improvements to other classes