Allow non-Text Controls in `ClassicGroupBox`'s `Header`

[BinToss]

On multiple occasions, I've wished to have an IsEnabled-bound CheckBox in a a GroupBox's Header.

Currently, ClassicGroupBox assumes Header is a string and returns null if it's anything else.

[alvahtin]

It is easy. Use Label instead of TextBox in Themes. And accordingly, Content instead of Text.