gui: Allow to open/save crosswords

Context

GUI part of #39 (closed):

• A new welcome screen to open a saved puzzle
• A save button in the editor view to save puzzle

Limitations:

• No way to edit puzzle metadata: To be done in subsequent MR(s)
• No way to delete a saved puzzle: To be done in subsequent MR(s)
• Save button is always active and doesn't indicate if there is unsaved changes: To be done with #76

GUI Puzzle Selection

(Note: Recording lagged a bit while showing puzzle search - application was actually fluid.)

What has changed?

Main Changes

gui-view-model

• Added a PuzzleSelectionViewModel

gui-view

• Created a welcome screen allowing to create a new grid or open a puzzle from the puzzle repository
• Added a save button to the crossword editor toolbar

gui-controller

• Added a controller to control calls to the PuzzleService, i.e.:
  • List available puzzles
  • Load a given puzzle
  • Save a given puzzle

gui-presenter

• Implemented PuzzlePresenter

gui

• Instantiate welcome screen
• Switch to editor view upon new puzzle or open puzzle button action

Side Effects

gui

• Renamed classes
• Moved error popup control logic out of crossword editor controller

Others

gui-view

• Added icons on lateral titled pane titles
• Changed dictionaries list view placeholder icon

gui-*

• Minor Javadoc improvements