[Checkbox] onEnabled/onDisabled setting misspelled or regressed

[petruavram]

I managed to track down the cause of the bug. It is a regression that was introduced by fixing #3148. I don't think it's possible to fix this and avoid the backwards incompatibility unless both forms of the methods are defined as settings and called inside enable.

Reproduced bug jsFiddle

[jlukic]

Thanks for the ping, I'll take a look

[jlukic]

Added both callbacks until 3.0 to avoid issues. Typos suck for backwards compatibility..
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