Merge pull request #525 from proski/master

Git related patches

Thank you.
I will have to look into the code again soon, because it is not general enough (too tied to thinking about Git only in terms of the Frescobaldi application while it should actually be used for managing LilyPond projects...). I think your improvements will be helpful there too.
typo in changelog

Closes #526
comment out object editor for 2.17 release
make object editor and audio export hidden in non-git releases
as per issue #514
activate new mainwindow, fix #314
a new settings option: enable experimental features
use "experimental features" pref in some places
document new experimental features feature
pot and nl.po update
change Nederlandese into Olandese
italian update by Federico Bruni
add argparse to the list of included modules in windows standalone build
Use universal_newlines=True with subprocess.Popen()

Remove conversion of output to str, subprocess.Popen() does it for us.
Read data in binary more if it's decoded manually

Python 3 would decode contents automatically if binary mode is not
specified.
Use new-style version argument parser, needed for Python 3 support
Eliminate bare print, even in commented out code

Use "from __future__ import print_function" when print() has multiple
arguments.
Fix old-style "raise", needed for Python 3 support
Use new style exec() for Python 3 compatibility
Import explicitly from current directory for Python 3 compatibility

This patch only fixes the imports that prevent Frescobaldi from starting
under Python 3.
Use empty string instead of True as groups key

Python 3 doesn't allow comparing bool and str. Empty string ensures that
"Lilypond Version" goes on top of the Insert menu.