Provide a way to define mandatory group of properties

AFAIU, the `PropertyGroup` annotation is here mostly to guide presentation and the `PropertyFeature` is more meaningful semantically.

Two barely related properties can completely be in the same group.

See an example of both annotations used at https://github.com/xwiki/xwiki-platform/blob/fb8e06afa1a7e46e33ce311eb4b09be81b0567f6/xwiki-platform-core/xwiki-platform-display/xwiki-platform-display-macro/src/main/java/org/xwiki/rendering/macro/display/DisplayMacroParameters.java#L66-L67

Group: reference + type of the reference – their content fills up different purposes, the purposes work well together so we decide to group them in the UI.
Feature: reference + page – their content have the same purpose (setting both would be overkill and we'd have to answer the question - which one to pick? ), they are not always next to each other.

IMO it makes more sense to make a feature mandatory rather than a group. From what I understand from the issue description, this would be very repetitive with the feature proposed in XCOMMONS-2738 .

Sources: Javadoc + https://extensions.xwiki.org/xwiki/bin/view/Extension/Properties%20Module#HPopulateaJavaBean

Closing the issue as a duplicate of XCOMMONS-2738. If I mistook the intention here (e.g. a mandatory group is one where every single property is mandatory), please reopen it