Creation of class Position, Grid and Main
Update of class ClickToShoot and Target

Position class is the model of the position of a target. It is used to
represent the X and Y coordinates of a target.

Grid class is the model of a grid which allows the creation of the
principal grid.

Main class allows to run a game.

ClickToShoot class update allows to create a grid, update the score, and
start the game.

Target class update allows to create a target with X and Y random
coordinates.