DevTools: introduce TargetBreakpoints as a presentation of breakpoint and its state within target

Source/devtools/front_end/sdk/Target.js

Index: Source/devtools/front_end/sdk/Target.js
diff --git a/Source/devtools/front_end/sdk/Target.js b/Source/devtools/front_end/sdk/Target.js
index 5fd831a0c6c3b7b0c736501cfb8a498d1972b8f3..d3fb0f9abb4ba1d7bec6c9aff6609aa1d3b87708 100644
--- a/Source/devtools/front_end/sdk/Target.js
+++ b/Source/devtools/front_end/sdk/Target.js
@@ -231,11 +231,19 @@ WebInspector.TargetManager.prototype = {
      */
     observeTargets: function(targetObserver)
     {
-        WebInspector.targetManager.targets().forEach(targetObserver.targetAdded.bind(targetObserver));
+        this.targets().forEach(targetObserver.targetAdded.bind(targetObserver));
         this._observers.push(targetObserver);
     },
 
     /**
+     * @param {!WebInspector.TargetManager.Observer} targetObserver
+     */
+    unobserveTargets: function(targetObserver)
+    {
+        this._observers.remove(targetObserver);
+    },
+
+    /**
      * @param {!InspectorBackendClass.Connection} connection
      * @param {function(!WebInspector.Target)=} callback
      */