Widgets: Make sure WP_Widget constructor creates a correct id_base value for a namespaced widget class.

The `id_base` value is used for the widget's `id` and `class` attributes and also for the option name which stores the widget settings (unless the widget specifies a custom `option_name` value).

With this change, any backslashes in the `id_base` for a namespaced widget class are converted to hyphens, making it easier to style the output or target the widget with JavaScript.

This also avoids a `preg_match(): Compilation failed` PHP warning from `next_widget_id_number()` on the Widgets screen, previously caused by unescaped backslashes.

Props Mte90, hermpheus, rogerlos, welcher, SergeyBiryukov.
Fixes #44098.

git-svn-id: https://develop.svn.wordpress.org/trunk@50953 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/widgets/custom-widget-classes.php

@@ -0,0 +1,14 @@
+<?php
+namespace Test\Sub\Sub {
+
+	class Namespaced_Widget extends \WP_Widget {
+	}
+
+}
+
+namespace {
+
+	class Non_Namespaced_Widget extends \WP_Widget {
+	}
+
+}