Feature/generic menu and lvgl implementation

[FiveTeethless]

This pull request basically resolves #2.

What it can do:

• generic rendering of a list of menu items
• SubMenu items can be clicked and another menu is displayed
• Switch items can modify a boolean variable, from which the pointer was supplied
• Value items can call a value modification screen, where the it can be changed via 3 buttons

What is still missing:

• the implementation of variable manipulation should rather work with callbacks instead of direct variable modification
• there should be a menu item to trigger a callback to other parts of the SW