Install GTK+ 3

Author: SiekamCebule

.github/workflows/ci.yml

@@ -27,6 +27,9 @@ jobs:
 
     - uses: seanmiddleditch/gha-setup-ninja@master
 
+    - name: Install GTK+ 3 development libraries
+      run: sudo apt-get install -y libgtk-3-dev
+
     - name: Build Linux Release
       run: flutter build linux --release