Added a Heightmap Heatmap display
A new widget "heightmap" has been added that grabs /sys/heightmap.csv
from the Duet and creates a simple 2D heat map and statistics.
dueui_config_default.json was updated with a new "HeightMap" page
inserted between Movement and Console.
I'm somewhat embarassed by the implementation but it does work
and it doesn't require any new third-party libraries.
Fixed issue where GCode responses weren't being shown
GCode responses for long running commands (like bed levelling)
are now being displayed correctly.
The "get_reply" parameter of the "gcode" action is now ignored
since all gcode replies are automatically displayed.