let fileinfo use the new ly.docinfo module

todo: merge documentinfo and fileinfo, based on ly.document and ly.docinfo
add DocInfo.global_staff_size()
better support iteration over all blocks
add version_tuple()
inherit from ly.docinfo to add some frescobaldi-specific stuff such as variables
load ly.parse
let documentinfo the new ly.docinfo module

todo: merge functionality more with fileinfo and autocomplete.harvest
also let fileinfo use the Frescobaldi-inherited lydocinfo
gradually move to the new ly.document infrastructure
add ly.docinfo.DocInfo.range

to perform searches on a subset of a document.
also let autocomplete use lydocinfo
let open file at cursor use lydocinfo and allow selecting multiple files
remove ly.parse