Completely rewritten keyword completion implementation using another approach

#KT-2816 Fixed

idea/testData/completion/keywords/InClassScope.kt

@@ -3,51 +3,26 @@ class TestClass {
 }
 
 // EXIST:  abstract
-// ABSENT: annotation
-// ABSENT: as
-// ABSENT: break
-// ABSENT: by
-// ABSENT: catch
+// EXIST:  annotation
 // EXIST:  class
-// ABSENT: continue
-// ABSENT: default
-// ABSENT: do
-// ABSENT: else
 // EXIST:  enum
-// ABSENT: false
 // EXIST:  final
-// ABSENT: finally
-// ABSENT: for
 // EXIST:  fun
-// EXIST:  get
-// ABSENT: if
-// ABSENT: import
-// ABSENT: in
+// EXIST:  in
+/*why?*/
+// EXIST:  inner
 // EXIST:  internal
-// ABSENT: is
-// ABSENT: null
 // EXIST:  object
 // EXIST:  open
-// ABSENT: out
+// EXIST:  out
+/*why?*/
 // EXIST:  override
-// ABSENT: package
 // EXIST:  private
 // EXIST:  protected
 // EXIST:  public
-// ABSENT: ref
-// ABSENT: return
-// EXIST:  set
-// ABSENT: super
-// ABSENT: This
-// ABSENT: this
-// ABSENT: throw
 // EXIST:  trait
-// ABSENT: true
-// ABSENT: try
-// EXIST:  typealias
 // EXIST:  val
 // EXIST:  var
-// ABSENT: vararg
-// ABSENT: when
-// ABSENT: where
-// ABSENT: while
+// EXIST:  vararg
+/*why?*/
+// NUMBER: 20