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.

Adjust some imgui windows to work better with different font sizes.

Introduce a new "Enable Time Tracking" checkbox in the InterfacePane UI. The checkbox is dynamically enabled or disabled based on the emulation state, preventing changes while emulation is active.

In StringUtil.h, the lambdas wrapping `Common::ToLower(char)` and `Common::ToUpper(char)` were only necessary due to the function names being overloaded.

Most of these changes are to improve consistency in capitalization.

Also clean up signals a bit.