Rename isPadding and isSpacing to just padding and spacing

[mvysny]

Kotlin naming convention seems to be not to use the "is" prefix in parameter arguments, e.g.

• String.startsWith(prefix: String, ignoreCase: Boolean = false): Boolean
• expect fun String.replaceFirst(oldValue: String, newValue: String, ignoreCase: Boolean = false): String

Or

public expect fun ByteArray.decodeToString(
    startIndex: Int = 0,
    endIndex: Int = this.size,
    throwOnInvalidSequence: Boolean = false
): String

Do this for verticalLayout{} and horizontalLayout{} and revisit all DSL functions as well.