Properly handle content on blank pages

[th-we]

There are two problems at the moment:

• all content from blank pages is stuffed into <front>, even for blank pages that are in the middle or at the end
• If there are multiple different measures followed by a (single) blank page, then objects on all of those pages have the OnNthBlankPage property set to 1. Sibmei treats everything that has the same OnNthBlankPage as belonging to the same page. We also have to take into account the bar number to differentiate between the different pages.

Content at the end can go into a <back> element, content in the middle can be stored in a <div> within the <section>, like so:

</measure>
<pb/>
<div>
    <!-- Content goes here -->
</div>
<pb/>
<measure xml:id="m-128" n="9">

(Blank pages can occur for multiple reasons and don't imply a section end. Therefore continue within the <section> element.)