DueUI v3.0.2 Released

Added UI Test Mode

* There's a new settings option "Test Mode" which, when turned on,
  will bypass attempts to connect to a Duet or SBC.  This will
  allow you to design the GUI without having access to an actual
  Duet device.  You must still provide a valid URL to the DueUI
  config file.

Heater widget updates

* Heater state values were updated for RRF 3.01

* Fixed incorrect heater tolerance calculations