Fixed keywords after 'this@' completion + fixed implementation of JetExpressionWithLabel.getLabelName()

idea/idea-completion/testData/handlers/smart/QualifiedThisKeywordName1.kt

@@ -4,5 +4,5 @@ class `this` {
     }
 }
 
-// ELEMENT: "this@this"
+// ELEMENT: "this@`this`"
 

idea/idea-completion/testData/handlers/smart/QualifiedThisKeywordName1.kt.after

@@ -1,8 +1,8 @@
 class `this` {
     fun String.foo(){
-        val foo: `this` = this@this<caret>
+        val foo: `this` = this@`this`<caret>
     }
 }
 
-// ELEMENT: "this@this"
+// ELEMENT: "this@`this`"
 

idea/idea-completion/testData/handlers/smart/QualifiedThisKeywordName2.kt

@@ -9,4 +9,4 @@ fun foo(){
             })
 }
 
-// ELEMENT: this@fun
+// ELEMENT: this@`fun`

idea/idea-completion/testData/handlers/smart/QualifiedThisKeywordName2.kt.after

@@ -4,9 +4,9 @@ fun bar(handler: Int.() -> Unit){}
 fun foo(){
     `fun`({
                 bar({
-                            val s: String = this@fun<caret>
+                            val s: String = this@`fun`<caret>
                         })
             })
 }
 
-// ELEMENT: this@fun
+// ELEMENT: this@`fun`