[TASK] Add preselection to SingleSelect

[BastianBalthasarBux]

Add the defaultValue option, supported by form viewhelper also to SingleSelect of ext:form, so one could define a preselected value of a select-field.

[TYPO3IncTeam]

Thank you for your contribution to TYPO3. We are using Gerrit Code Review for our contributions and took the liberty to convert your pull request to a review in our review system.
You can find your patch at: https://review.typo3.org/54078
For further information on how to contribute have a look at https://docs.typo3.org/typo3cms/ContributionWorkflowGuide/