History for SheetProviders

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!SheetProviders are PlugIns that provide DataSkins with property sheets.  They are informed by their DataManagers whenever a dataskin wants to add, delete, or access a property sheet which matches a name or XML namespace that the !SheetProvider has registered as supporting.  This allows, for example, the creation of !SheetProviders which add a row in an SQL database whenever a certain propertysheet is added, update it when the sheet is changed, and remove it when the sheet is deleted.

!SheetProviders give their DataManagers a list of the sheet names and a list of the XML namespace URL's that they support.  The special name '"*"' can be used in either list to mean that the !SheetProvider can be contacted for any property sheet name or namespace, respectively.

The !SheetProvider API is designed to allow multiple providers the chance to provide a given sheet.  This is in contrast to the API for AttributeProviders, which allows an attribute name to be mapped to only one !AttributeProvider.

Currently, SheetProviders' support for generating DataManagementEvents is non-existent, due to the fact that the propertysheets themselves are really the ones that need to generate them.  This will probably be solved in ZPatterns 0.5.0 by making the propertysheets DataSkins themselves, with the !SheetProviders acting as their DataManagers.  But this concept has not been thought through yet.