Add caching implementation of UPower.Device properties interface

Instead of retrieving each relevant property individually with a
synchronous DBus call on each propertiesChanged signal from a watched
device (at least 2 on a laptop, battery and AC, more when e.g. battery
powered peripherals are present), fetch the properties just once during
startup (with a single bulk "GetAll" query), and use the values from
the propertiesChanged signal for implicit updates.

Skip creation of instances for individual Devices if the DisplayDevice
is present, as the properties are not used at all.

Remove the autogenerated UPower.Device interface, as it is only able to
fetch properties synchronously, one by one.