Convert setlists between popular app formats.

There are currently some limitations to the converter, detailed in the following table

App	source	notes	→ Mobilesheets .msf	→ forScore .4SS (xml)	→ Piascore .pssl
Mobilesheets	.msf	All the songs need to be PDFs and the setlist must be exported WITHOUT metadata, annotations, midi commands, or audio. There is WIP handle metadata or not, and to render other file types to pdf.	✅	✅	✅
forScore	.4SS (xml)	ignores bookmarks... otherwise works fine	✅	✅	✅
forScore	.4ss	This format is currently read-only. Watch out for correct song to pdf association in files exported without annotations from old versions of the app...	✅	✅	✅
PiaScore	.pssl	Currently only tested with setlists from modern versions of the app that use pdf files.	✅	✅	✅