Queue API calls (continuation of #411)

Created by: mguijarr

Now that PR #411 has been merged, implementation of the solution for the AJAX requests agreed at MAX IV last week has to be done:

• once a call regarding queue operation is made, UI is "blocked" (Queue part is disabled / or there is some busy indicator)
• when the call is finished (ACK reply from server received), the UI is released for user operation

I would say priority is quite high, in order to finish the work started with PR #411.