Allow configuration of checkbox ActiveField label class name

[antichris]

For no apparent reason (not apparent to me, that is) the author of ActiveField decided, that the class attribute of a checkbox label should not be configurable. I beg to differ.

Q	A
Is bugfix?	yes/no
New feature?	no/yes
Breaks BC?	possibly
Tests pass?	yes
Fixed issues	no issue opened (yet)

BC could get broken for those relying on class being ignored in labelOptions (no idea why would anyone want to do that, though).

I figured that opening an issue for discussion would take more time then just creating a PR that fixes the problem, since an existing pull request can be merged or rejected any time, while an issue would require someone to create a PR for it to be resolved anyway.

[samdark]

Since it's there from the very beginning mikehaertl/yii2@7364249#diff-37ce2b0674ccb8129c6a33875aec0bbfR177. @mikehaertl do you remember why the class is nulled in case of checkboxes and radios?

[mikehaertl]

The examples in the BS3 docs have no class set for checkbox labels. But if you don't null it, you inherit the default class control-label from yii\widgets\ActiveField, which could possibly result in unexpected effects.

But I agree that there's a problem if you want to explicitely set a class. I'm just not sure about a proper fix.

[antichris]

I am well aware that this can actually be worked around by passing the class name(s) in the config parameter for ActiveField::label(), but that's a workaround and I strongly believe in getting things done without workarounds.