Added 'change to backing field' fix

idea/testData/quickfix/variables/changeMutability/afterValWithSetter.kt

@@ -0,0 +1,5 @@
+// "Make variable mutable" "true"
+class A() {
+    var a: Int = 0
+        <caret>set(v: Int) {}
+}

idea/testData/quickfix/variables/changeMutability/beforeValWithSetter.kt

@@ -0,0 +1,5 @@
+// "Make variable mutable" "true"
+class A() {
+    val a: Int = 0
+        <caret>set(v: Int) {}
+}