Create MEF analyzer that ensures exports are placed on supported types

[kzu]

For example, you could put a [Theme] attribute on a type (or property) that is not
of the expected target type ColorScheme, and this would only "fail" at run-time with
an invalid cast exception, which is clearly a bad experience.

The goal of this analyzer is to make the ThemeAnalyer general-purpose, so it can
diagnose any ExportAttribute-derived attribute and provide compile-time errors.