Log Message:
gtksourceview4: update to 4.2.0.

News in 4.2.0, 2019-03-15
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* New syntax highlighting definition file for: Fish, GDScript, Solidity,
  Dart, and Powershell.
* Improvements to the syntax highlighting of: reStructuredText, C++,
  GAP, CSS, HTML, JavaScript, Groovy, Meson, Python3, and Logtalk.
* Fix testsuite for changes in GLib hashtable ordering.
* Avoid double entry of languages based on globs.
* MSVC build improvements.
* Fix usage of GRegex for offset in text, which fixes a Valgrind warning.
* Ensure CSS is inherted from parent style schemes.
* Avoid storing GtkTextIter across main-loop boundaries.
* Fix incorrect transfer annoation for gtk_source_buffer_create_source_mark().
* Various memory leak fixes.
* Translation updates.