Fallback for dashboard widgets in custom manager theme (#16705)

### What does it do?
Loads the dashboard widget templates from the default theme, if they
don't exist in the custom manager theme.

### Why is it needed?
The
[documentation](https://docs.modx.com/current/en/building-sites/client-proofing/custom-manager-themes)
states that "The Manager will fall back to the default theme templates
for any that have not been overridden."
This is currently not the case for the dashboard widgets (that load a
Smarty template).

### How to test
* Create a new folder in `manager/templates` for a custom manager theme
and change the system setting `manager_theme`.
* **DON'T** create copies of the files in
`manager/templates/default/dashboard` in your own custom theme folder.
* Make sure the dashboard widgets get displayed on the manager welcome
page.

### Related issue(s)/PR(s)
Related topic in the MODX forum:
https://community.modx.com/t/manager-theme-customization-problem/8261

manager/controllers/default/dashboard/widget.buttons.php

@@ -21,7 +21,7 @@ public function render()
             $this->modx->smarty->assign($key, $value);
         }
 
-        return $this->modx->smarty->fetch('dashboard/buttons.tpl');
+        return $this->controller->fetchTemplate('dashboard/buttons.tpl');
     }
 
 

manager/controllers/default/dashboard/widget.configcheck.php

@@ -35,7 +35,7 @@ public function render()
         $this->modx->getService('smarty', modSmarty::class);
         $this->modx->smarty->assign('warnings', $response->getObject());
 
-        return $this->modx->smarty->fetch('dashboard/configcheck.tpl');
+        return $this->controller->fetchTemplate('dashboard/configcheck.tpl');
     }
 }
 

manager/controllers/default/dashboard/widget.grid-online.php

@@ -40,7 +40,7 @@ public function render()
         $this->modx->smarty->assign('data', $data);
         $this->modx->smarty->assign('can_view_logs', $this->modx->hasPermission('logs'));
 
-        return $this->modx->smarty->fetch('dashboard/onlineusers.tpl');
+        return $this->controller->fetchTemplate('dashboard/onlineusers.tpl');
     }
 }
 

manager/controllers/default/dashboard/widget.grid-rer.php

@@ -43,7 +43,7 @@ public function render()
         $this->modx->smarty->assign('data', $data);
         $this->modx->smarty->assign('can_view_logs', $this->modx->hasPermission('logs'));
 
-        return $this->modx->smarty->fetch('dashboard/recentlyeditedresources.tpl');
+        return $this->controller->fetchTemplate('dashboard/recentlyeditedresources.tpl');
     }
 }
 

manager/controllers/default/dashboard/widget.updates.php

@@ -60,7 +60,7 @@ public function render()
             $this->modx->smarty->assign($key, $value);
         }
 
-        return $this->modx->smarty->fetch('dashboard/updates.tpl');
+        return $this->controller->fetchTemplate('dashboard/updates.tpl');
     }
 }