Bugfix/workaround - due to the non-guaranteed ordering of property listener...

Bugfix/workaround - due to the non-guaranteed ordering of property listener callbacks, not all SliceCanvas objecst were updating their colour textures on changes to range clipping. You can now add a 'preNotifyFunc' to PropertyBase objects, which will be called as soon as a property value changes, before any listeners. A few other minor commentings and refactorings