Replace {Model,Value}HistorySource with HistoryProxySource to reduce code duplication
This deprecates {Model,Value}HistorySource and replaces it with a single HistoryProxySource that can collect history from any other source. This reduces duplicate code between the two sources. It also adds a "fillMode" property that can be used to indicate how HistoryProxySource deals with empty items.
Edited by Arjen Hiemstra