Sendable functionality broken for both Scheduler and CommandScheduler

[calcmogul]

Scheduler (old command framework) shows up on dashboard as an empty item with no state. CommandScheduler shows up with ids and names arrays which are both empty regardless of what's actually scheduled, and without any Cancel array (which should be there).

Neither are able to be displayed as any Shuffleboard widget other than NetworkTableTree.

The reason they don't display nicely is that they were never added as a widget.

https://github.com/wpilibsuite/shuffleboard/tree/master/plugins/base/src/main/java/edu/wpi/first/shuffleboard/plugin/base/widget

See wpilibsuite/allwpilib#2246 for context.