Introduction of sealed classes

Sealed classes can be derived only by their own inner classes or objects.
Their constructors cannot be called explicitly, so compiler knows all their descendants.
Incompatible modifier checks (final, abstract). Impossible with interface, object, enum.
A pack of tests provided.

idea/idea-completion/testData/keywords/AfterClassProperty.kt

@@ -35,4 +35,5 @@ class MouseMovedEventArgs
 // EXIST:  init
 /*why?*/
 // EXIST:  companion object
+// EXIST:  sealed
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/AfterClasses.kt

@@ -38,5 +38,6 @@ class B {
 // EXIST:  vararg
 /*why?*/
 // EXIST:  companion object
+// EXIST:  sealed
 /*TODO*/
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/AfterFuns.kt

@@ -38,4 +38,5 @@ class A {
 // EXIST:  init
 /*why?*/
 // EXIST:  companion object
+// EXIST:  sealed
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/FileKeyword.kt

@@ -20,6 +20,7 @@
 // EXIST: {"lookupString":"protected","itemText":"protected","attributes":"bold"}
 // EXIST: {"lookupString":"public","itemText":"public","attributes":"bold"}
 // EXIST: {"lookupString":"reified","itemText":"reified","attributes":"bold"}
+// EXIST: {"lookupString":"sealed","itemText":"sealed","attributes":"bold"}
 // EXIST: {"lookupString":"val","itemText":"val","attributes":"bold"}
 // EXIST: {"lookupString":"var","itemText":"var","attributes":"bold"}
 // EXIST: {"lookupString":"vararg","itemText":"vararg","attributes":"bold"}

idea/idea-completion/testData/keywords/GlobalPropertyAccessors.kt

@@ -38,5 +38,6 @@ var a : Int
 // EXIST:  vararg
 /*why?*/
 // EXIST:  companion object
+// EXIST:  sealed
 /*TODO*/
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/InClassBeforeFun.kt

@@ -36,4 +36,5 @@ public class Test {
 // EXIST:  init
 /*why?*/
 // EXIST:  companion object
+// EXIST:  sealed
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/InClassScope.kt

@@ -30,4 +30,5 @@ class TestClass {
 // EXIST:  constructor
 // EXIST:  init
 // EXIST:  companion object
+// EXIST:  sealed
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/InTopScopeAfterPackage.kt

@@ -29,5 +29,6 @@ package Test
 // EXIST:  vararg
 /*why?*/
 // EXIST:  companion object
+// EXIST:  sealed
 /*TODO*/
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/PropertyAccessors.kt

@@ -34,4 +34,5 @@ class Some {
 // EXIST:  constructor
 // EXIST:  init
 // EXIST:  companion object
+// EXIST:  sealed
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/PropertyAccessors2.kt

@@ -34,4 +34,5 @@ class Some {
 // EXIST:  constructor
 // EXIST:  init
 // EXIST:  companion object
+// EXIST:  sealed
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/PropertySetter.kt

@@ -36,4 +36,5 @@ class Some {
 // EXIST:  constructor
 // EXIST:  init
 // EXIST:  companion object
+// EXIST:  sealed
 // NOTHING_ELSE

idea/idea-completion/testData/keywords/TopScope.kt

@@ -28,5 +28,6 @@
 // EXIST:  vararg
 /*why?*/
 // EXIST:  companion object
+// EXIST:  sealed
 /*TODO*/
 // NOTHING_ELSE