t3onoff breaks radio fieldsets

[cheesegrits]

The current code for the t3onoff buttons breaks any such fieldsets (containing two radio inputs with 0 and 1 as values) that don't have the input before the label.

I've submitted a PR:

#515

... to use the 'for' property of the label to get the input, rather then relying on the DOM structure.
using prev() .

Note that the existing code already uses the 'for' property in the listener a few lines later, so already relies on there being a 'for'.