[new feature] DropdownMenu: add close-on-click-outside prop (#3824)

src/mixins/click-outside.ts

@@ -11,9 +11,16 @@ export type ClickOutsideMixinConfig = {
 
 export const ClickOutsideMixin = (config: ClickOutsideMixinConfig) =>
   Vue.extend({
+    props: {
+      closeOnClickOutside: {
+        type: Boolean,
+        default: true
+      }
+    },
+
     data() {
       const clickOutsideHandler = (event: Event) => {
-        if (!this.$el.contains(event.target as Node)) {
+        if (this.closeOnClickOutside && !this.$el.contains(event.target as Node)) {
           (this as any)[config.method]();
         }
       };