The Open in External Editor button makes editing a game INI file easy,
and the Refresh button is only useful if you edit the game INI. You're
not supposed to edit read-only game INIs, so let's not show these two
buttons for read-only game INIs.

I implemented this by hiding the buttons instead of not creating them in
the first place so we don't have to branch on whether the buttons exist
in GameConfigEdit::ConnectWidgets.

Back in 2018, a Presets system was added to DolphinQt's GameConfigEdit.
Presets was a dropdown menu where you could select a particular setting
to add to your custom game INI. Only a small number of settings were
made available, with the intent that more would be added over time.

8 years later, the set of available settings hasn't been expanded at
all, and I don't know of anyone who uses these presets. On top of this,
we have now made good progress in exposing per-game settings
graphically. I think the Presets system is best off removed.

In place of the Presets menu, we now have "Refresh" and "Open in
External Editor" buttons. These more useful actions were previously
hidden away in the Presets menu.

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.

Qt 5.0 introduced QRegularExpression to replace QRegExp. In Qt 6.0,
QRegExp is removed entirely in favor of it.

Provides the same behvaior, but in a much more concise manner.

DolphinQt: Replace unnecessary QStringLiterals with alternatives where applicable