Move KnownExtensions out of recipe to individual hosts #117
Description
KnownExtensions.cake contains a static class KnownExtensions with the definitions, including current version numbers, of all known extensions used in development and testing.
Keeping this as a common class implies that all hosts will use the same extension versions. This is not necessarily true and, in any case, we only have one current host, the GUI runner. The definition will be moved to the GUI repository and the file renamed for the other class it contains, ExtensionSpecifier.