In-place editing supports creating objects

The in-place editor for object properties introduced in XWiki 12.3 now also supports creating objects by specifying data-object-policy="updateOrCreate" in the <dt>-tag. The also-introduced velocity macro #createObjectAndPropertyIfMissing allows to (temporarily) add an object and a property to the current document using a reference to the property like "Some.UsefulClass[5].status" which is useful for displaying the default value of the property when the object doesn't exist in the document.