layout.phtml: Add hidden iframe for non-xhr file uploads

refs #8758

application/layouts/scripts/layout.phtml

@@ -54,6 +54,7 @@ $iframeClass = $isIframe ? ' iframe' : '';
 <div id="layout" class="default-layout<?php if ($showFullscreen): ?> fullscreen-layout<?php endif ?>">
 <?= $this->render('body.phtml') ?>
 </div>
+<iframe id="fileupload-frame-target" name="fileupload-frame-target"></iframe>
 <!--[if IE 8]>
 <script type="text/javascript" src="<?= $this->href($ie8jsfile) ?>"></script>
 <![endif]-->

public/css/icinga/layout-structure.less

@@ -47,6 +47,10 @@ html {
   }
 }
 
+#fileupload-frame-target {
+  display: none;
+}
+
 #responsive-debug {
   font-size: 0.9em;
   font-family: Courier new, monospace;