Fix unnecessary parenthesis around 'this' in refactorings

Operator priority calculation for 'this' expressions was incorrect in
JetPsiUtil.getPriority(): it would return a very low value for
a JetThisExpression, causing the IDE to report that parenthesis
are necessary in an expression like '(this)[1]'.

Fix this by making JetThisExpression not implement
JetStatementExpression by removing the implements clause from
JetLabelQualifiedInstanceExpression and adding it to all other
subclasses of JetLabelQualifiedInstanceExpression.

Fixes KT-4513.

idea/testData/intentions/removeUnnecessaryParentheses/unnecessaryParentheses8.kt

@@ -0,0 +1,6 @@
+class C() {
+    fun get(i: Int) = i + 1
+
+    // KT-4513: Unnecessary parenthesis around 'this'
+    val foo = (th<caret>is)[41]
+}

idea/testData/intentions/removeUnnecessaryParentheses/unnecessaryParentheses8.kt.after

@@ -0,0 +1,6 @@
+class C() {
+    fun get(i: Int) = i + 1
+
+    // KT-4513: Unnecessary parenthesis around 'this'
+    val foo = this[41]
+}