Fonts are now embedded in diagram, wiki, etc PDF exports.

A lot of general cleanup/refactoring for the old "Diagrams to PDF"
export wizard backend implementation. Consolidated some page numbering
code into org.simantics.export.core.pdf.PageNumbering.

Also consolidated iText FontMapper initialization into
org.simantics.export.core.pdf.FontMapping. It turned out that the key to
getting iText to embed fonts for text rendered into PDFs was that the
FontMapper used needs to have all system fonts registered into it.

Also fixed PDF export page number positioning in the more generic PDF
export wizard.

refs #7084

Change-Id: Ife4f96889659834a780566152f63e9e1ccd4979f