Native RadioTile required validation not supported

[brunnerh]

Currently a selection of a radio tile cannot be enforced using native form validation. To use this the same name has to be set on all radio inputs along with the required attribute.

Suggested changes:

• name should be propagated from TileGroup to RadioTile. It makes no sense for the elements of the group to have different names; they would no longer be part of the same group. The name property would then be removed from RadioTile.
• required should also be propagated down.

I implemented the logic, but currently there is still an upstream layout issue. (The validation message is displayed in the wrong place.)

[brunnerh]

Addressed by:

• fix(tile-group): add name and required props #1818