Don't crash if we encounter a field list.
Don't leave a pending node after writing an xref.

Previously a pending node was added but never removed. So if you had
two references on a slide, they would get mixed up with each other.

There's still a bug causing duplicate reference writing somewhere.
Use dimensions of placeholders on master slide.

Allows the user to control where text is placed, by editing the layout
of the master slide of the template presentation.

Load the dimensions from odplib/data/styles.xml if there's no template
presentation, instead of hard-coding them.