The current baobab code is in a bad way, so after some discussion on
IRC, we've decided that a rewrite is in order.

 - the core application logic will be switched to Vala

 - more effective use will be made of GtkBuilder

 - the charts (ringchart, treemap) and progress cell renderer are
   well-written, so we will keep the C versions of them around

 - the port will make use of GtkApplication, including GMenu

This commit represents the very start of a rather large
work-in-progress.