GTK sample (#515)

* GTK sample

* Add full UTF8 recoding ability to interop

Interop/Runtime/src/native/kotlin/kotlinx/cinterop/NativeUtils.kt

@@ -4,18 +4,7 @@ import konan.internal.Intrinsic
 
 internal fun decodeFromUtf8(bytes: ByteArray): String = kotlin.text.fromUtf8Array(bytes, 0, bytes.size)
 
-fun encodeToUtf8(str: String): ByteArray {
-    val result = ByteArray(str.length)
-
-    for (index in 0 .. str.length - 1) {
-        val char = str[index]
-        if (char.toInt() >= 128) {
-            TODO("non-ASCII char")
-        }
-        result[index] = char.toByte()
-    }
-    return result
-}
+fun encodeToUtf8(str: String): ByteArray = kotlin.text.toUtf8Array(str, 0, str.length)
 
 @Intrinsic
 external fun bitsToFloat(bits: Int): Float