Make the generic ValueConverter methods virtual

[ritijain]

Recently ran into a use case where a user might want to perform some sort of application specific pre-processing or post processing on the item mapping key-value pairs .

Example - the keyvalue pair could be an enum -> string mapping, but if we want a localized string, the string mapped result might need app specific post processing.

[ryantrem]

Seems reasonable. Though for the specific example you give above, in the past I've seen IValueConverter implementations that just aggregate other IValueConverters into a pipeline. So in your case above, you'd have a converter that went from an enum to a string (resource key), and then another converter that went from a resource key to a resource. Regardless, I think making these virtual seems pretty harmless and enables some new scenarios.