Add a dbus interface to trigger the bluetooth server

[mzanetti]

The dbus interface can be called with

qdbus io.nymea.networkmanager /io/nymea/networkmanager enableBluetoothService

It behaves the same as the push button

Also:

• fixes the button config not working at all (the button GPIO was set after creating the button on -1)
• get rid of loggingcategories.h (it doesn't scale and we're getting rid of it in nymead too)
• change Core to not be a singleton to enforce cleaner code (prevent Core::instance() being accessed from random places which encourages spaghetti code in the long run)