Solving issues with new language save

This is a common work with @Achal-Aggarwal.

It solves https://github.com/joomla-projects/com_localise/issues/198
It also prevents the new saved language to have the wrong client displayed on redirect after Save. 
Also it makes the client field readonly preventing issues when an edited language xml already exists for that client.

To test: create new language xml for different clients after setting the Languages view settings to different values for client, including all clients. Save (Not Save and Close) and check that the xml is the correct one displayed.

Yeah, we can do that also. But actually our aim is to not populate state after setState (called at L384).
we are setting language.id in this save method and in its parent save method a getState is called upon 'language.id' which again populates state. The only flag to stop populating state is $this->__state_set which is only modified in getState after calling populateState.