XMLAppCast.ParseAppCast should return the list of items

[Deadpikle]

Found an old note in the docs for that function:

NOTE TO SELF: In a new version of NetSparkle with breaking changes, for more flexibility, this should probably return the list of parsed items rather than setting a member value.

May want to consider modifying the IAppCastHandler interface, too, although this function doesn't currently appear there.

GetAvailableUpdates should probably take a List of AppCastItem, and this func probably belongs in another class as it has nothing XML specific. (We should probably rework the API to separate the app cast handling from the parsing for 3.x)