Removed unsafe eval() from compressed treeselectmenu script

referring to issue #3837:

> I'm using CSP on my server. Sadly, I have to allow unsafe-eval because of Joomla. There are more secure alternatives to eval():
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Don.27t_use_eval.21
See also http://www.w3.org/TR/CSP/#script-src
Oops, this was planed as a separate patch in #3844 
However, the old variant will fail with CSP enabled, because it still uses eval(). The new one works fine, with a fallback to eval().