Windows GUI: Use QMenu font (Segoe UI) for entire application

[nyanpasu64]

On Windows, Qt's default system font (MS Shell Dlg 2) is outdated.

Dolphin previously used over 15 lines of code to compute a font closer to the proper font, but with an approximately correct font size. Using the QMenu font directly is both more concise and more elegant (in my opinion).

Successor to #8359.

Credit to https://stackoverflow.com/a/40380684 for discovering this method. Is it worth crediting in the source code? I think not?

[CookiePLMonster]

Please provide before/after comparison screenshots.

[nyanpasu64]

Please provide before/after comparison screenshots.

It seems to be visually indistinguishable in the English locale, for fonts at the default size. There may be slight differences in other locales, or parts of the UI using differently sized fonts (but I can't find any, even in the GC/Wiimote controller viewers).

Master:

PR:

TBH I'm mostly doing this as a proof of concept. Dolphin already has modern fonts on Windows, so this is primarily a code cleanup. However many other Qt-based programs are still stuck on MS Shell Dlg 2, and would benefit more from this method. (Credit to https://stackoverflow.com/a/40380684 for discovering this method.)

[MayImilae]

Wow that looks exactly the same. I guess it isn't something I need to worry about!

[spycrab]

LGTM apart from one minor note.
(Just to be sure I tried it on a HiDPI Monitor. No differences there either.)

[nyanpasu64]

QApplication::font("QMenu") traces down to https://code.woboq.org/qt5/qtbase/src/plugins/platforms/windows/qwindowstheme.cpp.html#505 which uses lfMenuFont. I don't know if I can really document this in the code though. (Sidenote: Qt's font handling is special, extremely complex, and has some unexpected properties especially on KDE.)