markdown: Introduce Markdown support for task descriptions
This commit adds a Markdown parser to task descriptions. Each GtdTaskListView contains one Markdown parser, that is then shared between the rows.
The Markdown parser keeps a weak reference every GtkTextBuffer added, so it can update a map of buffers that were populated with the appropriate tags.
Issue: #157 (closed)
Edited by Georges Basile Stavracas Neto