News in 3.20.0
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* Improvements to the syntax highlighting of: CSS, JavaScript, protobuf, Vala,
imagej, Python 3 and BibTeX
* Adapt code to use GTK_TEXT_VIEW_LAYER_BELOW_TEXT and
GTK_TEXT_VIEW_LAYER_ABOVE_TEXT
* Add a "sourceview" CSS class to all GtkSourceView's
* Fixes to the MSVC build system
* Various other small fixes and improvements
* Translation updates
