replace optparse with argparse

optparse is deprecated with Python 2.7.
This *should* replace it without changes to the behaviour.
But as I don't have experience using Frescobaldi with different
command line arguments I could not test it thoroughly.
small argparse usage improvements
argparse messages
new pot, po, nl translation
Merge pull request #499 from wbsoft/argparse

replace optparse with argparse
require python 2.7
reword action Close All Documents to say that it also leaves the session

close #500
small i18n update
fix #501 incorrect name of setting: set-path -> set-paths
Merge pull request #502 from dliessi/issue-501

fix #501 incorrect name of setting: set-path -> set-paths
[Snippets] Allow selection for relative music
Merge pull request #504 from PeterBjuhr/snippets

[Snippets] Allow selection for relative music
allow Fraction and DecimalValue in the toplevel context
hopefully fix #503
midiinput: more robust unloading
Merge pull request #507 from PeterBjuhr/musicxml

Musicxml Export wip
Merge pull request #505 from mamoch/master

midiinput: more robust unloading
dont try to load non-existent file when traversing an included file
fix remote with argparse
Updated french translation
Merge pull request #510 from rdoursenaud/french

Updated french translation