gui: Introduce end-to-end tests

Context

This initiates end-to-end tests for GUI. It relies on TestFx.

Only a few tests have been added. These tests suffice to make the overall code coverage bump from 66% to 81% because:

• A lot of the GUI code is run when application launches.
• Only view-model was tested until now.

What has changed?

Main Changes

gui-tests

• Created module.
• Added a first test.

tests-coverage

• Added gui-tests to the JaCoCo report aggregation.

Side Effects

.gitlab-ci.yml

• Updated build image: JavaFx has additional run-time native dependencies, even in headless mode.
• List of added dependencies:
  • liberation-fonts: Some fonts; Without them, JavaFx refuses to start because it cannot find any font.
  • pango: Bundled JavaFx native library libjavafx_pango.so requires it.
• Image is now very big (532 MB).