Fix ToggleSplitButton's IsCheckedProperty not being a public DP

[Felix-Dev]

Description

This PR adds a public DP IsCheckedProperty to the ToggleSplitButton.

Motivation and Context

Fixes #2218.

How Has This Been Tested?

Tested locally.

Open Questions

1. Do I need to add an annotation for the added DP (like [WUXC_VERSION_PREVIEW])?
2. Do we need to add a test here? I added an initial test to check if Binding works (no matching interaction test yet). It follows the enabled/disabled ToggleSplitButton test on the same MUXControlsTestApp page which also has no accompanying interaction test (1). Is it intentional that there is no interaction test for it?

(1)

[StephenLPeters]

Technically, I think this would need API review... @MikeHillberg thoughts?

[StephenLPeters]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephenLPeters]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ranjeshj]

@Felix-Dev thanks for fixing this. Can you add a quick api test in https://github.com/microsoft/microsoft-ui-xaml/blob/master/dev/SplitButton/APITests/SplitButtonTests.cs ?

You can just create the button, set the property, get it and check its value. You could also use SetValue/GetValue to validate.

[Felix-Dev]

@ranjeshj Added an API test. Should I remove the button I added to the MUXControlsTestApp (see image in opening post)?

[ranjeshj]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ranjeshj]

Technically, I think this would need API review... @MikeHillberg thoughts?

Following up on this. This should be a quick review, so no worries about it.

[StephenLPeters]