new method to get commandline for lilypond-tool
use new toolcommand() method
let musicxml import use the lilypondinfo
keep using the full path for the importfile.

also, dont use os.chdir (it changes workdir globally)
use subprocess.Popen (although later we will use QProcess and have a "live" log window")
let musicxml2ly write to stdout and then set the text and url of the document, but not write it to disk yet