Enhance: Allow choosing a light or dark appearance instead of following the system #714

GitX takes its appearance entirely from System Settings. NSAppearance+PBDarkMode only ever reads effectiveAppearance, nothing sets NSApp.appearance, and no preference or menu item exists for it, so there is no way to keep GitX light while the rest of the Mac is dark, or the other way round.

Asked for in #662 (comment).

Proposal: a three-way setting - System (the default), Light, Dark - stored as a preference and offered both in Settings > General and as a View > Appearance submenu.

Setting NSApp.appearance is the mechanism, with nil meaning "follow the system". That cascades to every window, sheet and web view, and it updates effectiveAppearance, so the existing PBEffectiveAppearanceChanged plumbing and every consumer of it keep working unchanged. The legacy WebView resolves prefers-color-scheme from its effectiveAppearance too, so the blame and fileview stylesheets follow the override as well.