Fix #6043 Added support for array type styles and classes

[klimov-paul]

Fix #6043 Added support for array type styles and classes.
BC kept.

Pay attention for unit test HtmlTest::testMergeCssClass(): it demonstrates the solution for issues like:
#8229
#4916

yiisoft/yii2-bootstrap#15

[samdark]

Nice!

[alexandernst]

Nice to see this :)
Should I close #8229 @cebe @klimov-paul ?

[klimov-paul]

First of all, the whole team should accept the approach.
Then we need to decide, which key should be used for classes added by Yii.
I whould like to use 'widget' for the classes added by widgets.

[alexandernst]

Attention ping @yii-devs :)
This is something quite important, imho, so the sooner it gets discussed and merged, the better.

[samdark]

Well, I like the approach. @qiangxue, @cebe thoughts?

[cebe]

looks great to me :)

[klimov-paul]

Relates: yiisoft/yii2-bootstrap#52