added a button manager system which eases the creation of menus and enables control via keyboard in menus
documented ManagedButton and made it posible to instantiate it without a position and set the position afterwards
modified button positioning behavior of ButtonManager
documented ButtonManager
adapted main menu for use with the button manager
documented ButtonManager::calculateButtonPos() and removed parameter layout because it was redundant
implemented ButtonManager::removeButton()
various improvements to ButtonManager

- Support of the left/right keys in horizontal menus
- fixed bugs in coherence with button repeat of return key
- modified button delay/repeat settings
adapted all remaining menus to use the button manager instead of the old buttons
fixed crucial bug in class Highscore

A new #1 Highscore caused a segfault.
fixed year in copyright notice
removed now useless class Button from the project
edited README