Lock display on startup mask when klippy is not ready

[seho85]

The Display will now show only the "Startup Mask" when Klippy is not ready.

The Startup Mask:

• shows the Klippy State
• the Klippy Message
• has a button for "Restart"
• has a button for "FW Restart"

For the implementation the WebsocketInterface was extended that changes of the klippy state can be subscribed.

Also changed startup routine that a proper error message is now shown when reading on of the configuration files has failed.

Fixes #16