made keymap public (#4053)

* made keymap public
* better docs

src/mixins/itext_key_behavior.mixin.js

@@ -34,9 +34,17 @@ fabric.util.object.extend(fabric.IText.prototype, /** @lends fabric.IText.protot
   },
 
   /**
-   * @private
-   */
-  _keysMap: {
+   * For functionalities on keyDown
+   * Map a special key to a function of the instance/prototype
+   * If you need different behaviour for ESC or TAB or arrows, you have to change
+   * this map setting the name of a function that you build on the fabric.Itext or
+   * your prototype.
+   * the map change will affect all Instances unless you need for only some text Instances
+   * in that case you have to clone this object and assign your Instance.
+   * this.keysMap = fabric.util.object.clone(this.keysMap);
+   * The function must be in fabric.Itext.prototype.myFunction And will receive event as args[0]
+   */
+  keysMap: {
     9:  'exitEditing',
     27: 'exitEditing',
     33: 'moveCursorUp',
@@ -50,17 +58,17 @@ fabric.util.object.extend(fabric.IText.prototype, /** @lends fabric.IText.protot
   },
 
   /**
-   * @private
+   * For functionalities on keyUp + ctrl || cmd
    */
-  _ctrlKeysMapUp: {
+  ctrlKeysMapUp: {
     67: 'copy',
     88: 'cut'
   },
 
   /**
-   * @private
+   * For functionalities on keyDown + ctrl || cmd
    */
-  _ctrlKeysMapDown: {
+  ctrlKeysMapDown: {
     65: 'selectAll'
   },
 
@@ -77,11 +85,11 @@ fabric.util.object.extend(fabric.IText.prototype, /** @lends fabric.IText.protot
     if (!this.isEditing　|| this.inCompositionMode) {
       return;
     }
-    if (e.keyCode in this._keysMap) {
-      this[this._keysMap[e.keyCode]](e);
+    if (e.keyCode in this.keysMap) {
+      this[this.keysMap[e.keyCode]](e);
     }
-    else if ((e.keyCode in this._ctrlKeysMapDown) && (e.ctrlKey || e.metaKey)) {
-      this[this._ctrlKeysMapDown[e.keyCode]](e);
+    else if ((e.keyCode in this.ctrlKeysMapDown) && (e.ctrlKey || e.metaKey)) {
+      this[this.ctrlKeysMapDown[e.keyCode]](e);
     }
     else {
       return;
@@ -109,8 +117,8 @@ fabric.util.object.extend(fabric.IText.prototype, /** @lends fabric.IText.protot
       this._copyDone = false;
       return;
     }
-    if ((e.keyCode in this._ctrlKeysMapUp) && (e.ctrlKey || e.metaKey)) {
-      this[this._ctrlKeysMapUp[e.keyCode]](e);
+    if ((e.keyCode in this.ctrlKeysMapUp) && (e.ctrlKey || e.metaKey)) {
+      this[this.ctrlKeysMapUp[e.keyCode]](e);
     }
     else {
       return;