Implement performance overlay support

* performance overlay and fps counter are now configurable over ls2 again so we can
  disable/enable them from the settings app
* states are stored only per session and not accross restarts

Signed-off-by: Simon Busch <morphis@gravedo.de>
Merge pull request #135 from webOS-ports/performance

Implement performance overlay support
Fix binding issue with performance overlay

* when user disables the overlay by clicking on it there the binding as it is implemented
  doesn't work and the values from the overlay and the service will be out of sync causing
  us not being able to activate the overlay again.

Signed-off-by: Simon Busch <morphis@gravedo.de>