Checked declaring class internally whether or not it is Kotlin class …

[serkan-ozal]

…while inserting after method body

[chibash]

Including isKotlinClass() in insertAfter() causes an extra overhead even when the class is a Java class. So I added isKotlin() to CtClass and leaved insertAfter() without a call to isKotlinClass(). I also added how to call isKotlin() when calling insertAfter() to the javadoc.
Note that not only the Kotlin compiler generates the bytecode we need to the redundant insertAfter() but also a Java compiler such as Eclipse's may do.