Using the quickly ubuntu-application template (i.e. python and GTK3), what solutions exist to display a text editor in which I can highlight some parts of the text and add annotations/extra visuals at some places?
The idea is to have a component where the user can type in text and as he does so there are some annotations schown (similar to syntax highlighting).
The editor does not have to be a rich text editor or anything like an office application, just a text editor would be fine.
The two most-used text widgets are Gnome's native text widget, and Scintilla. Each do rich text but are often used for plain text. I'm sure they both handle highlighting, can be programmed to include annotation and comment text, and are able to incorporate graphic objects.
GTKText is the text widget in PyGTK; see http://www.pygtk.org/pygtktutorial/ch-textwidget.html et al.
Scintilla is used widely in GTK and Windows, at least; here it is: http://www.scintilla.org/ in its own website.
Exactly how this is used with Quickly, I'm not certain -- but if are writing GUI code for a Quickly app, these links will at least give you an idea of what's out there.
Best of luck!