Add gtk tests

Currently we only test the GtkBuilder files.
Also I can't find a way to get gtk to uninitialize and reinitialize
in a different thread.

Close #56

scripts/test.sh

@@ -26,7 +26,7 @@ xvfb-run -a -s "-screen 0 1024x768x24" \
     --env=CARGO_HOME="target/cargo-home" \
     --env=CARGO_TARGET_DIR="target_test/" \
     app ${MANIFEST_PATH} \
-    cargo test -- --test-threads=1
+    cargo test -j 1 -- --test-threads=1
 
 # Create a flatpak bundle
 # flatpak-builder --finish-only app ${MANIFEST_PATH}