Prevent smartSelectExpression from breaking up 'this@outerClass'

The previous commit made JetThisExpression not implement
JetStatementExpression. This had the side effect that
smartSelectExpression over a label-qualified this expression
(e.g 'this@outerClass') would now also list a plain 'this'
as a potential expression. Restore the old behaviour by adding an
explicit check for JetThisExpression.

idea/testData/smartSelection/labelledThis.kt

@@ -0,0 +1,9 @@
+class Foo() {
+    val s = th<caret>is@Foo.toString()
+}
+
+// 'this' should not be listed, only 'this@Foo'
+/*
+this@Foo
+this@Foo.toString()
+*/