Theme editor: VirtualJoystick is not available as an Item Type

### Tested versions

- Reproducible in: 4.7-dev1, 4.7-dev2.
- Not applicable prior to 4.7 (new feature).

### System information

Godot v4.7.dev2 - Ubuntu 24.04.4 LTS 24.04 on Wayland - X11 display driver, Multi-window, 1 monitor - OpenGL 3 (Compatibility) - Mesa Intel(R) HD Graphics 620 (KBL GT2) - Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz (4 threads) - 15.50 GiB memory

### Issue description

`VirtualJoystick` does not appear in the Add Item Type list in the theme editor, despite that it has theme properties.

Note that if you manually type "VirtualJoystick" and press Add Type, it does successfully connect up to the real `VirtualJoystick` node type, and provides access to the node's theme properties (`ring_normal_color`, `ring_pressed_color`, etc). It is just missing from the list and search functionality.

### Steps to reproduce

1. Create a `Control`, set a `theme` to a new theme.
2. In the Theme editor, press the "+" button next to Type.
3. Note that `VirtualJoystick` does not appear in the list (and a search for "Virt" shows nothing).

### Minimal reproduction project (MRP)

N/A

Reproduce the issue in Godot 4.7-dev2 by creating a Control with a new theme and opening the Theme editor's Add Item Type list. Trace how the list and search are populated for VirtualJoystick, then verify that it appears in both and still exposes its theme properties such as ring_normal_color and ring_pressed_color.

Written by the indexing model from the issue text.