Up/Down arrow keys in numerical fields should adjust value

[danielgtaylor]

This lets you easily adjust the value without typing numbers, and is especially useful for small changes. Modifiers could be used to change larger amounts (e.g. up = +0.01, alt+up = +0.10, shift+up = +1.00). Need to check on cross-platform key bindings and see what's available.

May need to keep plain up key as selecting the start of the string, and use modifiers for this new functionality.