make it possible to set icons for custom device plugin columns

plugins/DevicePlugins/Columns/DevicePluginColumn.php

@@ -17,5 +17,9 @@ use Piwik\Plugin\Dimension\VisitDimension;
  */
 abstract class DevicePluginColumn extends VisitDimension
 {
-
+    /**
+     * Can be overwritten by Columns in other plugins to
+     * set a custom plugin not included in Piwik Core
+     */
+    public $columnIcon = null;
 }
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