[Form] Added check for parent disabled status in Button form elements

The Button form element did not check for the parent disabled configuration status,
making them behave differently to all other form widgets.
[Form] Added test for disabling buttons
bug #10591 [Form] Buttons are now disabled if their containing form is disabled (webmozart)

This PR was merged into the 2.3 branch.

Discussion
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[Form] Buttons are now disabled if their containing form is disabled

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10109
| License       | MIT
| Doc PR        | -

Commits
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ebfee72 [Form] Added test for disabling buttons
6bb355e [Form] Added check for parent disabled status in Button form elements