Removing option for temporary layer

Instead, adding setter and getter for a private temp_ property,
as suggested by @elemoine.

src/ol/layer/vectorlayer.js

@@ -303,8 +303,9 @@ ol.layer.Vector = function(options) {
   /**
    * True if this is a temporary layer.
    * @type {boolean}
+   * @private
    */
-  this.temp = goog.isDef(options.temp) ? options.temp : false;
+  this.temp_ = false;
 
 };
 goog.inherits(ol.layer.Vector, ol.layer.Layer);
@@ -343,6 +344,14 @@ ol.layer.Vector.prototype.clear = function() {
 };
 
 
+/**
+ * @return {boolean} Whether this layer is temporary.
+ */
+ol.layer.Vector.prototype.getTemporary = function() {
+  return this.temp_;
+};
+
+
 /**
  * @return {ol.source.Vector} Source.
  */
@@ -604,6 +613,14 @@ ol.layer.Vector.prototype.setRenderIntent = function(renderIntent, features) {
 };
 
 
+/**
+ * @param {boolean} temp Whether this layer is temporary.
+ */
+ol.layer.Vector.prototype.setTemporary = function(temp) {
+  this.temp_ = temp;
+};
+
+
 /**
  * @param {Array.<ol.Feature>} features Features.
  * @return {string} Feature info.