Fixel tool

Mostly waiting for Dave to give this a test-run and make sure everything works as expected, but there are some points of contention that others may like to comment on:

* In the fixel tool, I've put the colourmap menu alongside the intensity scaling buttons. I admit it's more cramped than previous, but it makes more sense from a UI perspective: those features are only valid if colouring by value, so apart from better grouping those features, if this isn't the case it disables the whole row to show that those settings currently have no effect.

* I initially thought the AdjustButtons didn't have a deadzone because I kept dragging them by accident. So I've doubled the deadzone size. If there's contention about this one we can make it a config file option instead.

* If the technique I've used for handling the tristate buttons in the fixel tool is useful elsewhere as well, might be worth encapsulating it.