drawspaces plugin: more flexible configuration
drawspaces plugin: more flexible configuration.
The configuration in GtkSourceView is a matrix, it's no longer a simple list of flags. Take advantage of that.
For the design (both of the code and UI), see:
https://wiki.gnome.org/Projects/GtkSourceView/SpaceDrawing
The SpaceDrawerChooser can be implemented in the gedit plugin as a first step before putting it in a library.