Package Kit: Implement progress notifications.

- BJobStateListener: Add progress state and corresponding hook.
- FetchFileJob: Notify job progress hook on libcurl notifications.
- UserInteractionHandler: Add hooks for download progress and checksum
  validation progress.
- PackageManager: inherit from JobStateListener and watch for job
  notifications for internally generated jobs. Forward to corresponding
  UserInteractionHandler hooks as needed.
- Adapt pkgman, HaikuDepot and package_daemon to above changes.
  Neither HaikuDepot nor package_daemon's progress hooks are wired up to
  do anything yet though.

headers/private/package/FetchFileJob.h

@@ -29,6 +29,10 @@ public:
 									const BEntry& targetEntry);
 	virtual						~FetchFileJob();
 
+			float				DownloadProgress() const;
+			const char*			DownloadURL() const;
+			const char*			DownloadFileName() const;
+
 protected:
 	virtual	status_t			Execute();
 	virtual	void				Cleanup(status_t jobResult);
@@ -46,6 +50,7 @@ private:
 			BString				fFileURL;
 			BEntry				fTargetEntry;
 			BFile				fTargetFile;
+			float				fDownloadProgress;
 };