Enhancement: Add mandatory fields

[bfiessinger]

At the moment there is no possibility to set fields to mandatory. It would be great if there was a checkbox to choose whether the field is required or not.

[nk-o]

Added experimental Required option for top-level controls (inside repeater is not supported).

Further improvements:

• [ ] Show error notice after the user clicked on the Publish button. For now, we can only lock the post from publishing, which is not user-friendly, because the user may not know what happened and why he can't save the post. Waiting for additional events or filters https://github.com/WordPress/gutenberg/issues/13413
• [ ] Add support for controls inside Repeater