Merging property data provider implementation

[grigasp]

There's a use case for showing properties of multiple data sources (imodels, but not necessarily just them) in a property grid. The property grid takes the properties through a IPropertyDataProvider interface. I believe the best approach would be to provide a function like createMergedPropertyDataProvider(providers: IPropertyDataProvider[]): MergingPropertyDataProvider. The merged provider should:

• Trigger onDataChanged event whenever any of the merged providers do that.
• Merge PropertyData objects from every provider into a single one. I imagine this may cause duplicate properties - that's okay, let's leave it up to consumers to ensure that there are no duplicates, either by nesting their property data under special categories or ensuring property labels are unique.
• Provide ability to find the source provider, given a PropertyRecord - for editing cases we'll need to know where the property came from.