CB-8139 WP8. Fix callback for plugins with native ui (capture, contactPicker, BarcodeScanner, other)

Unload event could not be used to detect when CordovaView is not used anymore. For example, this event is triggered when we execute command that shows some native elements on new page and then we return back. In this case Unloaded event is called but, but control state is preserved and we should continue to use that Cordova view instance.

This commit changes the following:
1. Allows command instances to be garbage collected => fixes corresponding memory leak so that NativeExecution is no more required to call DetachHandlers for each command. This fixes  CB-8139.
2. Use Loaded and Unloaded events to add/remove handlers for native events; this makes it possible for GC to destroy CordovaView when it is not required anymore