Livetable readiness detection for selenium tests based on XWIKI-7120 do not always works

Since not all livetable expose themselves in global variable, RightUI.vm for example only use a local variable, the detection of readiness does not works in all cases.

The patch provided here will only base detection on the DOM elements, and not passing through the livetable object to access the loading bar. This could also have a drawback if the loading bar is not properly identified, but this should work properly with the latest recommandation, which is to identify it with

-ajax-loader (and no more with just ajax-loader alone, which may cause collisions).