Avoid undefined:undefined style properties when disabling styles

Using the checkboxes in the rule-view to disable/enable properties actually creates new "undefined:undefined;" properties every time.
This change should hopefully fix this problem.

I also took the opportunity to look at why !important properties were not always handled correctly, but any fix I attempted lead to crazy range fix-up assertion errors.
I understand why we need to deal with ranges, but wasn't able to fully grasp the logic of the updateRange and adjustPoint functions while working on this bug.
A little guidance would be great.