JS update

I am sorry for breaking the tests, but the spinner is one tough enemy :)

* figured out the proper spinner detection (lightbox zoomed widget had made spinner_div to appear visible although it was not)
* moved whole spinner detection into JavaScript (faster)
* added "library file" that we can put our JS shortcuts in to not pollute python source.
* slight code cleanup (always use module's `execute_script`)
* fixed widget's footer getter (did not properly react on `Never` state)
What I meant was get rid of the global and actually load the file at import time so we fail quickly if the js library can't be found.

What do you think about functionalizing the ajax wait code and putting it in the JS lib?