Set the text display to 'internal' in headless environments

In headless without a DOM host environment text display is set to
'internal' to prevent access to the DOM in several text related methods
in renderer/abstract.
Export a global JXG variable in WebWorker environments

Since the implementation of the AMD pattern we only introduce our global
variable 'JXG' in special cases. To make JSXGraph available inside WebWorker
threads we have to somehow provide our main namespace. This is done
by introducing a new property called 'JXG' to the global self object.

Including JSXGraph in WebWorker threads is now done by e.g.

    importScripts('js/jsxgraphcore.js');
    var board = self.JXG.JSXGraph.initBoard(null);
Expose several Turtle methods and properties to JessieCode