Layout options

The layout option in the export dialog and the import preferences (see above) triggers writing and reading pb and sb elements.

When writing, all page and line (system) breaks are encoded into the MEI. The ones manually entered in MuseScore have a @type="mscore-manual".

When reading the MEI, only pb and sb with @type="mscore-manual are set as page and line breaks and all the others are ignored.

Relevant tests: