Automatically adjust default colors when theme changes

Set `BaseDefinition` to instances of `PredefinedClassificationTypeNames`.

`F# Functions` now inherits from `Identifier` so we have to change it once in Fonts and Colors dialog for testing.

Reference:
http://social.msdn.microsoft.com/Forums/vstudio/en-US/739639c1-c8d1-4d29-bfb0-5c8319c33599/default-colors-for-classification-based-on-theme?forum=vsx