handle the FileOpen request on mac OS X

it is implemented by installing an event handler on the global Application
object and looking for the QEvent.FileOpen type. (Please test!)
avoid too technical language, make spinboxes more clear
even smarter translatable prefix/suffix in spinbox
slightly more in style
handle FileOpen request after mainwindow has been created
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
docstring
some more gui text guidelines
remove unneeded imports; just ask for a tokens iterable
VCSRepo: Add abstract fetch() method