Allow users to modify the number of classes in manual interval symbology

[wayfaringrob]

Allow users to increase and more easily decrease the number of symbol classes. Currently the option to change the number of classes is grayed out. The only way to add new classes is to reset your symbology using another method and start over. This is extremely frustrating when you're trying to experiment with different symbology settings.

Further, reducing the number of classes (by removing them) does not update the color ramp.