Recently I was contacted by Gerhard Kalab with a wish to support syntax coloring in Notecase. We discussed the idea and the possible implementation, but since I didn't have much time, he implemented this himself.
You can find the sources here. This is a patched notecase v1.9.5 using GtkSourceView instead of the standard GtkTextView as a text displaying widget.
The bad news is that the GtkSourceView crashes when trying to apply the syntax coloring to a note that contains the pictures.
Hopefully, the future versions of this library will have this bug fixed, and the notecase will have both syntax coloring and a margin with line numbers implemented (supported by a new widget library).
Thanks Gerhard for your work!