U4-1949 Default document type properties to Textstring datatype

This pull request implements the functionality discussed on the issue tracker for U4-1949.

A new configuration key can be added to web.config to set the default document type property that will be used when adding new properties to a document type from the back office.  The key is:

<add key="umbracoDefaultDocumentTypeProperty" value="Textstring" />

The value is the text (case sensitive) of the type - e.g. "Textstring", "Richtext editor".

If the key is missing "Textstring" is used by default.