Convert member to extension: Support members of header/impl classes

#KT-18883 Fixed

idea/testData/multiModuleQuickFix/memberValToExtensionByImpl/header/header.kt

@@ -0,0 +1,3 @@
+header class Foo {
+    val foo: Int
+}

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

@@ -0,0 +1,4 @@
+header class Foo {
+}
+
+header val Foo.foo: Int