Menu is working correctly.

Board is not generated before level is started. This can be tested by
pressing 's'. This will set the game state of the Renderer to InGame, so
it draws the game screen while GameManager is stil in meu mode.

Menu is not done yet :
-More testing is needed.
-The code layout should probably be changed.
-Unable to go back to menu.
Change in gmae mode is now handled in Run()

Changes in game mode are now handled by MenuManager. MenuManager has a
bool that states wether the game state has changed. When this boos is
read from GameManager, this value will be set back to false, meaning it
will only be true the first time you check this boolean variable.

This means MenuManager handles everything related to the current state
and switching between them, which is better than it being handled by
both GameManager and MenuManager.
Renderer now uses MenuItem struct