Accomplished using `run-clang-tidy` with `performance-move-const-arg,performance-unnecessary-value-param,modernize-pass-by-value`.

Changed arguments to const references, removed them where inappropriate (e.g. sink parameters). Same with std::move.

Manually reviewed each change to make sure that it makes sense, and do something more appropriate if possible.

- Row height is no longer hardcoded to `24`; instead, the height is
  determined by the font height of the currently selected debug font.
- Header height has been reduced (a factor of the font height) as it
  only serves as a column resizer.
- A `OnDebugFontChanged()` slot has been connected to properly update
  the widget on debug font changes.

| Before | After |
| --- | --- |
| <img alt="Registers widget (before)" title="Registers widget (before)" src="https://github.com/user-attachments/assets/8a6644f9-1081-4ac8-9fff-c79ddb73e103" /> | <img alt="Registers widget (after)" title="Registers widget (after)" src="https://github.com/user-attachments/assets/67f799d0-8afd-4081-bb40-3d7a76ddc705" /> |

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Saves on CPU usage when pausing/unpausing with the debugger disabled.
This is especially important when using frame advance rapidly.