gui: Introduce button to delete puzzle in puzzle selector

Context

See #39 (closed).

New edit/delete buttons

What has changed?

Main Changes

spi-presenter

• Added methods to PuzzlePresenter to notify puzzle deletion.

core

• Call the new PuzzlePresenter upon successful deletions.

tests

• Updated puzzle tests.

gui-view

• Reworked WelcomeScreenPane:
  • The open puzzle button is now on the selected list cell itself.
  • Added a delete button.
  • Both buttons have custom shapes defined by an SVG path.

gui-controller

• Added a controller method for calling the PuzzleService#delete(long) method.

gui

• Call gui-controller upon event from gui-view.

cli

• Implement new PuzzlePresenter methods.
• Adjust tests.

Others

gui

• Cosmetic code improvements

tests

• Enable puzzle test suite so that it is automatically run (forgot to do it until now).