make ly.music accessible from within docinfo frescobaldi, using caching
every Item has a position; the positions are sorted for m.iter_depth()

this way we can quickly locate an Item in the music tree from a cursor position
dont cache the music doc in the include node
add methods to find the node or time at a certain cursor position, and to find the length of music between two cursor positions
typo
Merge branch 'master' of https://github.com/wbsoft/frescobaldi
remove accidental translation