[Feature Request] Have chemdispensers remember custom amounts

Created by: 9600bauds

It's useful to be able to dispense 15u at a time, but since the button clears when you select another option, you have to click Custom and type 15 every time. It'd be more useful if the "Custom" button remembered whatever amount you gave it, and clicking it twice or having a small "edit" button next to it or whatever to modify the custom amount.