Add MenuSystem library by Jon Black

License: LGPL v3
Hook in on the go menu system

The menu structure gets initialized and added
hooks to the switch handling and display routines.

No actions / display output implemented yet.
-> Not usable yet
Improve MenuSystem::back()

Let us know if we are in the root menu or not
Implement menu actions

Note: Bluetooth is switchable via the menu only!
Make menu system update the display immediately
Fancier menu strings
Activate menu action on DISP2 long press, too
MenuSystem library: Prevent memory overrun by adding too many items
Increase max number of items
Implement scrolling of long menus
Add readme for the menu system
Factor out switch actions into own functions
Implement complete customizable switch action
Enable backlight in the menu system
Bugfix: Reset LCD on change from 'important info' screen back to graphical screen

In fact, fix this in a generic way.

The soft poti status message was messing
up the graphical info screen.