Add watching support for installation location package changes

Can be requested/stopped via BPackageRoster::{Start,Stop}Watching().
The notification message has the what code B_PACKAGE_UPDATE and contains
fields "event", "location", and "change count".

headers/os/package/PackageRoster.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2011, Haiku, Inc.
+ * Copyright 2011-2014, Haiku, Inc. All Rights Reserved.
  * Distributed under the terms of the MIT License.
  */
 #ifndef _PACKAGE__PACKAGE_ROSTER_H_
@@ -34,6 +34,23 @@ class BRepositoryCache;
 class BRepositoryConfig;
 
 
+// watchable events
+enum {
+	B_WATCH_PACKAGE_INSTALLATION_LOCATIONS	= 0x0001,
+		// de-/activation of packages in standard installation locations
+};
+
+// notification message "event" field values
+enum {
+	B_INSTALLATION_LOCATION_PACKAGES_CHANGED,
+		// "location": int32
+		//		the installation location
+		//		(B_PACKAGE_INSTALLATION_LOCATION_{SYSTEM,HOME}
+		// "change count": int64
+		//		the installation location change count
+};
+
+
 class BPackageRoster {
 public:
 								BPackageRoster();
@@ -68,6 +85,10 @@ public:
 									BPackageInstallationLocation location,
 									BPackageInfoSet& packageInfos);
 
+			status_t			StartWatching(const BMessenger& target,
+									uint32 eventMask);
+			status_t			StopWatching(const BMessenger& target);
+
 private:
 			status_t			_GetRepositoryPath(BPath* path, bool create,
 									directory_which whichDir) const;