add etreeutil.py, intended for xml.etree.ElementTree util functions
encode xml text in utf-8 (as it also declares this encoding)
use etreeutil.indent to prettyprint xml instead of reparsing using xml.minidom

also changes tabs to spaces in the python code
musicxml export: correct encoding-date, add frescobaldi version
if FileOpen events occur directly on start, dont setCurrentDocument() them all, just do that later. Hopefully issue #244 is satisfyingly fixed now?
always install the FileOpen event handler

even if the app is sessionRestored (although I dont know whether Mac
OS X uses a session manager)
let ElementTree write the XML itself.
(oops)
fix error message
fix #248
upd chlog