ISDb: Move to own namespace.

Also move OpenURL() to SubtitleDlDlg, it was only ever used there and wasn't specific to ISDb.
ISDb: Support hashing using the IAsyncReader filter interface.

Adds support for RARFileSource.
PPageFileInfoDetails: Try to request the filename and size from the filter first.

Allows RARFileSource to show the real filename and size of the file being played.
WebClientSocket: Try to request filename and size from the filter.