Core: Let any thread call previously host-thread-only functions

Fusion is one of the built-in styles that Qt ships with, and that is
generally supported in all platforms and handles custom color palettes
properly.

The color palettes have been borrowed from the Dolphin Memory Engine
buddy application.

The new styles are:

- **Fusion Light**
- **Fusion Dark Gray**
- **Fusion Dark**

A demo of each style on Windows:

| System | Light | Dark | Fusion Light | Fusion Dark Gray | Fusion Dark |
| ------ | ----- | ---- | ------------ | ---------------- | ----------- |
| <img width="1920" height="1080" alt="Dolphin Emulator  System style" src="https://github.com/user-attachments/assets/7f55a19d-d9a1-43d1-a435-1e1d5b29abe2" /> | <img width="1920" height="1080" alt="Dolphin Emulator  Light style" src="https://github.com/user-attachments/assets/4c70f2f9-16b8-4777-b72b-55b2dffcd1e4" /> | <img width="1920" height="1080" alt="Dolphin Emulator  Dark style" src="https://github.com/user-attachments/assets/5e669477-d2a5-4d19-b2c5-a2ed9bb1e6fe" /> | <img width="1920" height="1080" alt="Dolphin Emulator  Light Fusion style" src="https://github.com/user-attachments/assets/b1f95c47-0691-4809-bd74-99e913c17684" /> | <img width="1920" height="1080" alt="Dolphin Emulator  Dark Gray Fusion style" src="https://github.com/user-attachments/assets/c9d30aa3-f941-4fc5-806f-d3fbd2cae0cc" /> | <img width="1920" height="1080" alt="Dolphin Emulator  Dark Fusion style" src="https://github.com/user-attachments/assets/123d2125-e126-4e8c-aa42-793ded8ffacc" /> |

> [!NOTE]
> Notice that the **Light** and **Dark** styles remain available only on
> Windows due to limitations on how styles in the various platforms
> handle (or mishandle) custom color palettes.

> [!IMPORTANT]
> Due to [`KDE-511547`](https://bugs.kde.org/show_bug.cgi?id=511547),
> after having used Breeze (default style in Plasma systems), top tool
> bars in tools areas will lose their correct background color; a
> restart will be required in order to get the updated color in the main
> window.
>
> UPDATE: The issue has been fixed upstream and should be available in
> KDE Plama 6.5.3 and newer.

The core no longer cares which thread is the host thread.

Cleaning up Android's HostThreadLock is left for another PR, in part
because the HostThreadLock in NativeConfig.cpp still serves a purpose,
and in part to make any issues easier to bisect.

DolphinQt: Fix Windows dark theme when using external stylesheets.

It is useful enough for the Code widget to be enabled the first time
debugging is enabled, but it should not be re-enabled every time the
setting is toggled off and then back on. This also ensures that the
interface is consistent if debugging is enabled without using the
checkbox.