[Feature #8] Navigation improved with pad and keyboard

The new navigation method is implemented for title screen only.
Basically, each button has a navigation map, with up, down, left or right as keys and the name of the next button as a value.
[Feature #8] Pause menu and Upgrade menu now uses the new navigation system.
[Feature #8] On upgrade screen, when the player moves to resume and moves back to the upgrades, it selects the last upgrade the user was on.
Merge branch 'feature/navigation-controls' into develop
[Feature #14] Credits added on title screen.
[Feature #2] Missile launcher keyboard controls removed and directional arrows can now control the laser!