Add Icon/Button to show user's open tasks

This shows an icon with the number of open tasks assigned to the user.
If the user has open tasks and they click on the icon, they should see a
table with all their open tasks. Another click on the icon hides the
table.

Known Issues:
-------------
* If a task is check-off, the number next to the icon is not updated
dynamically.

SPR-962

scripts/userTasksOverlay.js

@@ -0,0 +1,34 @@
+(function() {
+    'use strict';
+
+    var $userTasksButtons = jQuery('a.plugin__do_usertasks');
+    $userTasksButtons.click(function handleUserTasksButtonClick(event) {
+        var $this;
+        event.stopPropagation();
+        event.preventDefault();
+
+        if (jQuery('.plugin__do_usertasks_list').length) {
+            jQuery('.plugin__do_usertasks_list').toggle();
+            return;
+        }
+
+        $this = jQuery(this);
+
+        jQuery.get(
+            DOKU_BASE + 'lib/exe/ajax.php',
+            {
+                call: 'plugin_do_userTasksOverlay',
+            },
+        ).done(function showUserTasksOverlay(data) {
+            var $wrapper = jQuery('<div class="plugin__do_usertasks_list"></div>');
+            $wrapper.css({'display': 'inline-block', 'position': 'absolute'});
+            $wrapper.append(jQuery(data.html));
+            $wrapper.appendTo('#dokuwiki__header');
+            $wrapper.position({
+                my: 'middle top',
+                at: 'right bottom',
+                of: $this,
+            });
+        });
+    });
+})();