load: trim xml string if necessary (#790)
load: trim xml string if necessary (#790)

close #790

xml strings with extra whitespace / newlines at the beginning weren't loaded correctly.
this is technically not a bugfix, since there should be now whitespace at the beginning
of xml strings, but it's a favor to users, to prevent their errors.

opensheetmusicdisplay/opensheetmusicdisplay,opensheetmusicdisplay/opensheetmusicdisplay,opensheetmusicdisplay/opensheetmusicdisplay