Button refactor

Refactor buttons.py for common implementation of shared functionality. Should serve all common button types equally well, with modified/complex buttons needing to pull in the appropriate base classes. Some comments provided, but no full documentation.

Breaking changes are to ElevationBehavior (renamed to RectangularElevationBehavior) and RoundElevationBehavior (renamed to CircularElevationBehavior). Names chosen to match the ones used in RippleBehavior. Includes full refactor of all affected code within kivymd.