Validate aliases.

module/dca/tl_leaflet_layer.php

@@ -267,6 +267,7 @@ $GLOBALS['TL_DCA']['tl_leaflet_layer'] = array
                     ['title'],
                     \Netzmacht\Contao\Leaflet\DependencyInjection\LeafletServices::ALIAS_GENERATOR
                 ),
+                \Netzmacht\Contao\Leaflet\Dca\Validator::callback('validateAlias'),
             ),
             'eval'          => array('mandatory' => false, 'maxlength' => 255, 'tl_class' => 'w50', 'unique' => true),
             'sql'           => "varchar(255) NOT NULL default ''"