Make Text>>getSelectionRange side-effect free

[fbo]

... and shorten the method if possible.

In Firefox, the new selectionString method is enabled. Placing the caret at the end of an editable text and typing causes new characters to be inserted BEFORE the last character in that text.

This among the five most annoying things I've seen in Lively. Cost me at least a day.

Once this is done, clean up Text>>selectionString so it does not use domSelection().toString() any more, which is meant for debugging, apparently.

[fbo]

Still don't see it. Too weird. Call getSelectionRange in selectionString, and it breaks. Don't call it and it works. If you see anything that could have side effects on selection, ranges, ... please let me know.