Introduce Variable feature (in pre-alpha quality).

Problems:
1. Still problems with containers, due to new for me JetContainerNode.
2. Needs name suggester
3. Needs name validator
4. Needs type annotation adding
5. Needs changing to var and checking write access of usages
6. Needs final modifier adding
7. Some problems on class and file level. Should be fixed. Problems with inplace rename on this positions. Additionally possibly should be added proper work with expressions on class body level and namespace/file level. It parsed as error now.
8. Needs much more tests than just 6.

idea/testData/refactoring/introduceVariable/IfElseAddBlock.kt

@@ -0,0 +1,12 @@
+fun a() {
+    if (true) 2
+    else <selection>1</selection>
+}
+/*
+fun a() {
+    if (true) 2
+    else {
+        val i = 1
+    }
+}
+*/