Add callback handlers for LiveWindow

[ThadHouse]

No description provided.

[sciencewhiz]

Fixes #2223

[sciencewhiz]

I tested with an java old command project and made a command that sets a speed controller to a random number in execute. I added a call to the scheduler run in testPeriodic.

With 2020.1.2, when I went to test mode, the command continued to execute. With this PR, the command stopped executing when going to test mode as expected.

More testing with new commands and C++ to come

[sciencewhiz]

The New Command template calls CommandScheduler.getInstance().cancelAll(); in testInit. Should that be removed in this PR so it doesn't happen twice?

[sciencewhiz]

Tested New Command with Java and didn't have any problems.

[sciencewhiz]

Tested old command with C++ without issues

[sciencewhiz]

@Oblarg any heartburn about removing the cancelAll in the template if it's being done behind the scenes with this PR?

[Oblarg]

I think it's slightly better to do it explicitly but I don't feel super strongly about it.

[sciencewhiz]

Tested new command with C++ without issues