Add possibility to define icons for plugin-buttons

Change-Id: I4a30fa72953ae6136d5c5056c992bdadac135ffe
diff --git a/dev/scss/main/buttongroup.scss b/dev/scss/main/buttongroup.scss
index 49df159..b78ed88 100644
--- a/dev/scss/main/buttongroup.scss
+++ b/dev/scss/main/buttongroup.scss
@@ -132,4 +132,7 @@
   > span {
     @include blind;
   }
+  &[data-icon]::after {
+    content: attr(data-icon);
+  }
 }