Add callback contexts

Add the notion of a callback context. This enables callbacks
to have logic around the conditions they were invoked in.

There are two context on which call back can be invoked
1) Startup: during startup all the call backs will be called
2) Signal: As part of condition match on the watched properties.

Callback would behave differently based on the context.
eg: eventCallback
1) Startup: Don't take any action.
2) Signal: Create the Dbus Object for the event.

Change-Id: If455558798ac3e44bbd8a93de0ce1b09d2e308ae
Signed-off-by: Ratan Gupta <ratagupt@in.ibm.com>
diff --git a/src/propertywatch.hpp b/src/propertywatch.hpp
index 55f6e5b..cd286a5 100644
--- a/src/propertywatch.hpp
+++ b/src/propertywatch.hpp
@@ -50,7 +50,7 @@
          *
          *  Watch callback interface implementation for PropertyWatch.
          */
-        void callback() override;
+        void callback(Context ctx) override;
 
         /** @brief Update properties.
          *