Handle unicode text encoding and text escape sequences

* Fix echo() encoding of strings (#838)
* Add escape sequence handling \x \u and \U
* Fix error handling for unicode characters (#737)

ToDo:
* [x] Fix the NBSP test case (echotest_no-break-space-test)
* [x] Validate if https://github.com/openscad/openscad/compare/text-escape-sequences#diff-efe9f8f7d601fffd8bedc312e8713866R202 works on different endian => works on PPC64