Simplify the OPA element selection

Allow user to define the exact css selector he wants to use in the extension directly by typing. For example, "div.container img.toto" to get KPIs for images having the class "toto" contained in a div class=container.

Having an area like this to precisely say what we want to select would make it much easier and would help to get the elements not selectable when the code is not exactly working fine (such as on VW or jecorenov). Without this feature, some elements are impossible to select in the OPA.