Allow to import/export puzzles

Context

Completes #6 (closed). This adds import/export functions to Croiseur.

Here's how GUI now looks:

Screencast Import/Export

(Note: The typos in the "back to puzzle selection" button tooltip, visible on the screencast, have been fixed, see this comment).

What has changed?

Main Changes

common

• Added PuzzleCodecDetails: Metadata of an encoder/decoder

spi

• Added spi-puzzle-codec
• Added new presentation methods from import/export

core

• Added functions to list codecs
• Added functions to import/export puzzles

puzzle-xd-codec-plugin

• Created plugin adapting puzzle-xd-codec to spi-puzzle-codec.

cli

• Implemented the commands corresponding to the functions added to core

gui

• Implemented the view and controller corresponding to the functions added to core. In particular:
  • Added an Import button in welcome screen puzzle selector, which opens a file chooser to import a puzzle to the puzzle repository
  • Added an Export button in editor view puzzle pane, which opens a file chooser to export the last puzzle save to the puzzle repository

Side Effects

core

• Split the PuzzleService into PuzzlePersistenceService, PuzzleImportService and PuzzleExportService to reduce complexity.

Others

gui-tests

• Created the module, for end-to-end testing from GUI.

Reviewers

Code has already been reviewed through several smaller MRs (see #6 (closed) for the complete list). This last big MR gives the whole picture and is the occasion to see if everything fits. Hopefully, no big issue will be raised here.