webui: widget initialization

- used `ctor_init` instead of `init` to avoid name collision with
  existing logic
- `ctor_init` is called right after widget instantiation. Basically support
  better inheritance for the old class system which doesn't have proper
  contructors

https://fedorahosted.org/freeipa/ticket/4402

Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>

install/ui/src/freeipa/widget.js

@@ -212,6 +212,14 @@ IPA.widget = function(spec) {
     that.clear = function() {
     };
 
+    /**
+     * Widget post constructor/factory initialization
+     *
+     * Called by builder by default.
+     */
+    that.ctor_init = function() {
+    };
+
     /**
      * Set enabled state.
      * @param {boolean} value - True - enabled; False - disabled
@@ -5998,6 +6006,7 @@ exp.post_op = function(obj, spec, context) {
         nc.container = obj.widgets;
         builder.build('widget', spec.widgets, nc);
     }
+    if (obj.ctor_init) obj.ctor_init();
     return obj;
 };