Resource: Do not auto update the cache by default

The resource class generally connects to the ResourceWatcher, and
updates itself whenever any other application changes any data. While,
this may be desirable at times, most of the times, it is not required.

1. It creates a massive number of watches which increase the number of
messages that are sent across dbus.

2. It slows down the Resource class because its own changes are also
propogated back to it.

3. It doesn't really notify anyone that the data has been changed, so
the ui code doesn't know when it is supposed to refresh.

Added a function setWatchEnabled( bool ), which configures if the
Resource should be auto-updated.