Make font for sensor readings larger

[alexdewar]

The font size for the sensor readings is currently just the default (i.e. the same as for all the other components), but researchers have reported that it would be nice to be able to read these values more easily.

A naive approach would be to hardcode a larger font size for these GUI components, but I don't think this is the right one. It would be better to figure out what the default font size is (it could be set by the user systemwide) and express the larger font size as a proportion of this.