Completely rewritten keyword completion implementation using another approach

#KT-2816 Fixed

idea/testData/completion/keywords/AfterClassProperty.kt

@@ -1,7 +1,33 @@
 class MouseMovedEventArgs
 {
   public val X : int = 0
-  in<caret>
+  <caret>
 }
 
-// EXIST: internal
+// EXIST:  abstract
+// EXIST:  annotation
+// EXIST:  as
+// EXIST:  class
+// EXIST:  enum
+// EXIST:  final
+// EXIST:  fun
+// EXIST: get
+// EXIST:  in
+/*why?*/
+// EXIST:  inner
+// EXIST:  internal
+// EXIST:  object
+// EXIST:  open
+// EXIST:  out
+/*why?*/
+// EXIST:  override
+// EXIST:  private
+// EXIST:  protected
+// EXIST:  public
+// EXIST: set
+// EXIST:  trait
+// EXIST:  val
+// EXIST:  var
+// EXIST:  vararg
+/*why?*/
+// NUMBER: 23