Represents a custom document used by a
Custom documents are only used within a given
CustomEditorProvider. The lifecycle of a
managed by the editor. When no more references remain to a
CustomDocument, it is disposed of.
The associated uri for this document.
Dispose of the custom document.
This is invoked by the editor when there are no more references to a given
CustomDocument (for example when
all editors associated with the document have been closed.)