Add 'operator' Intention: Support header/impl functions

#KT-18851 Fixed

idea/testData/multiModuleQuickFix/addOperatorByHeader/header/header.kt

@@ -0,0 +1,5 @@
+// "Add 'operator' modifier" "true"
+
+header class Foo {
+    fun <caret>unaryMinus()
+}

idea/testData/multiModuleQuickFix/addOperatorByHeader/header/header.kt.after

@@ -0,0 +1,5 @@
+// "Add 'operator' modifier" "true"
+
+header class Foo {
+    operator fun unaryMinus()
+}

idea/testData/multiModuleQuickFix/addOperatorByHeader/js/js.kt

@@ -0,0 +1,5 @@
+impl class Foo {
+    impl fun unaryMinus() {
+
+    }
+}

idea/testData/multiModuleQuickFix/addOperatorByHeader/js/js.kt.after

@@ -0,0 +1,5 @@
+impl class Foo {
+    impl operator fun unaryMinus() {
+
+    }
+}

idea/testData/multiModuleQuickFix/addOperatorByHeader/jvm/jvm.kt

@@ -0,0 +1,5 @@
+impl class Foo {
+    impl fun unaryMinus() {
+
+    }
+}

idea/testData/multiModuleQuickFix/addOperatorByHeader/jvm/jvm.kt.after

@@ -0,0 +1,5 @@
+impl class Foo {
+    impl operator fun unaryMinus() {
+
+    }
+}