Use pandoc to convert freestanding markdown files to docbook for
inclusion in the generated docs, and use bits and pieces of
gtk-doc code to continue expanding typical gtk-doc abbreviations.
The new tool for markdown -> docbook is a python script called
gtk-markdown-to-docbook.

The markdown dialect is specified via a list of pandoc extension
in gtk-markdown-to-docbook. It includes header annocations,
definition lists and tables, among other things.

This commit converts the 3 overview chapters (drawing, input
handling and actions) and the migration guide to markdown
syntax. Other files that are still listed in content_files
can be converted later.

This commit adds a pandoc dependency.