basic setup corresponding to file import
First version
accidentals supported
Basic durations supported
tuplets supported
Merge branch 'master' of https://github.com/wbsoft/frescobaldi into export-musicxml

Conflicts:
	frescobaldi_app/menu.py
reverting export_colored_html
Merge branch 'master' of https://github.com/wbsoft/frescobaldi into export-musicxml
ties supported
handle different file structures
setting initial bar attributes per part
dots supported
fixing bar attributes at start
Merge branch 'master' of https://github.com/wbsoft/frescobaldi into export-musicxml
Merge pull request #252 from PeterBjuhr/export-musicxml

Basic musicXML export
VCSRepo: Add abstract fetch() method
Make _run_git_command() more flexible (error handling)

As it seems Git commands are quite 'versatile' with regards
to their output on stdout and stderr ;-)
Therefore it seems better not to raise exceptions in the
basic _run method, even if that means more and some
redundant code in the higher-level caller functions.

_run_git_command() now returns a tuple of
- the stdout result (formatted as a list of strings)
- the stderr output.

Calling functions should handle the presence of 'error'
themselves, deciding whether to raise an exception or not.
VCS: Cosmetic touch to git checkout
VCS: Implement git fetch

Currently only works when the user has configured git
so it doesn't need interaction (e.g. passwords) when
communicating with the remote server.

Simply passes stderr output to the caller, so the calling function
(e.g. in the GUI) has to determine if an 'error' is regular info
output or indicates an exception
VCS: Add differing_commits() method

Compare local with (already fetched) remote branches.
Return
- remote_branches (that aren't present locally)
- local_branches (that aren't present remotely)

Can be used to determine the need of updates