Add ability to add custom collections

[PeterKvayt]

Hi!

I need to add custom implementation of Collections property:

So right now I can only select one proptery editor for it:

This property editor has ui alias Umb.PropertyEditorUi.Collection and it is hardcoded here.
And this UmbInputCollectionConfigurationElement used by UmbDocumentTypeWorkspaceViewStructureElement .

It would be great if there is will be a possibility to add another property editor ui aliases to select as Collections property.

Thanks!