progress bar for loading screen
the function “view.setLoading()” should get an optional argument to show
the progress of the loading process as in
http://getbootstrap.com/components/\#progress
In addition an arbitrary text should be displayed. The function should
have the signature
view.setLoading(true/false, progress_in_percent, text=“”)
(from redmine: issue id 2235, created on 2015-06-06 by Christian.Glaser)