Slider improvements

**Is your feature request related to a problem? Please describe.**

There is no easy method of determining if the slider value has changed. Checking if the value has changed requires addition variables to store old and current values and a conditional statement.

**Describe the solution you'd like**

Slider should have a method such as ```was_changed```. Currently there are ```custom_formatter``` and ```custom_parser``` methods that allow changes to be post-processed. ```was_changed``` should allow for computations based upon the old stored value and the new (current) stored value.

**Describe alternatives you've considered**

Alternatives seem to be messy old and new value computations, perhaps in a separate helper function.

Start by locating the Slider implementation and reviewing the existing custom_formatter and custom_parser methods. Define how a was_changed method would expose the old and current stored values, then verify that callers can use it for change-dependent computations without keeping duplicate state.

Written by the indexing model from the issue text.