fix(5897) Add missing initialization of $templatable in BooleanAttribute

The merge in 1.3-dev took part of other changes without taking the
 initialization for this attribute, which messed things up.
Should be better like this.

issue #5897

include/simpleplugin/attributes/class_BooleanAttribute.inc

@@ -25,6 +25,7 @@ class BooleanAttribute extends Attribute
 {
   public $trueValue;
   public $falseValue;
+  protected $templatable = TRUE;
 
   /*! \brief The constructor of BooleanAttribute
    *