Text widget: bump the TinyMCE modals z-index to 500001 so they show in the Customizer.

Props greuben.
Fixes #41158 for trunk.
Built from https://develop.svn.wordpress.org/trunk@40990


git-svn-id: http://core.svn.wordpress.org/trunk@40840 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/tinymce/plugins/wordpress/plugin.js

@@ -1,7 +1,9 @@
 /* global getUserSetting, setUserSetting */
 ( function( tinymce ) {
 // Set the minimum value for the modals z-index higher than #wpadminbar (100000)
-tinymce.ui.FloatPanel.zIndex = 100100;
+if ( tinymce.ui.FloatPanel.zIndex < 100100 ) {
+	tinymce.ui.FloatPanel.zIndex = 100100;
+}
 
 tinymce.PluginManager.add( 'wordpress', function( editor ) {
 	var wpAdvButton, style,