Add removable backdrop templates for close-on-click-outside dialogs

Packages/viewerbase/components/basic/removableBackdrop/removableBackdrop.html

@@ -0,0 +1,7 @@
+<template name="removableBackdrop">
+    <div class="removableBackdrop noselect"
+        oncontextmenu='return false;'
+        unselectable='on'
+        onselectstart='return false;'>
+    </div>
+</template>