Add property to disable cursor change on FlatButton

[embarc-gabriel]

Resolves: audacity/audacity#8146

We have some situations where we want the button to be unresponsive.
We cannot use the enabled the functionality once this has other side effects like changing the opacity.
Apart from that, even if the enabled on the mouseArea is false we still can see the cursor shape to change once within the mouse area.

This PR is only a suggestion on how this could be achieved.

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