add "modes" configuration, now "presentation" and "offline".

add "modes" configuration, now "presentation" and "offline".

They are presented as a submenu with check items. Need to implement
support in modules, so they'll check for these variables.

DESIGNERS: could you please provide the following icons:

   * preferences-modes: contain the other icons

   * preferences-modes-presentation: system is in presentation mode,
     no screensaver, locks and dpms will be available.

   * preferences-modes-offline: system is offline, modules that keep
     polling internet services (weather, forecasts, ...) should stop
     and show "I'm offline" emblem. When network systems (network
     manager, connman, exhalt...) are integrated, they will change
     this state as well.

SVN revision: 40375