Add properties hasUndo and hasRedo to State (#193)

* Add properties hasUndo and hasRedo to State

* Fix description fo hadRedo

docs/reference/models/state.md

@@ -22,6 +22,8 @@ For convenience, in addition to transforms, many of the [`Selection`](./selectio
   - [`fragment`](#fragment)
   - [`inlines`](#inlines)
   - [`texts`](#texts)
+  - [`hasUndo`](#hasundo)
+  - [`hasRedo`](#hasredo)
 - [Selection-like Properties](#selection-like-properties)
   - [`{edge}Key`](#edgekey)
   - [`{edge}Offset`](#edgeoffset)
@@ -97,6 +99,15 @@ Get a list of the lowest-depth [`Inline`](./inline.md) nodes in the current sele
 
 Get a list of the [`Text`](./text.md) nodes in the current selection.
 
+### `hasUndo`
+`Boolean`
+
+Whether there are undoable events.
+
+### `hasRedo`
+`Boolean`
+
+Whether there are redoable events.
 
 ## Selection-like Properties
 

lib/models/state.js

@@ -58,6 +58,26 @@ class State extends new Record(DEFAULTS) {
     return 'state'
   }
 
+  /**
+   * Is there undoable events?
+   *
+   * @return {Boolean} hasUndo
+   */
+
+  get hasUndo() {
+    return this.history.undos.size > 0
+  }
+
+  /**
+   * Is there redoable events?
+   *
+   * @return {Boolean} hasRedo
+   */
+
+  get hasRedo() {
+    return this.history.redos.size > 0
+  }
+
   /**
    * Is the current selection blurred?
    *