Support freedesktop dark style preference

[idoric]

I don't know how qalculate-gtk is supposed to behave, so I don't know if I'm writing a bug report or an improvement proposal.

Qalculate-gtk doesn't follow the freedesktop dark style preference, neither during operation nor at startup, although the "Theme" option is set to "Default". Concretely, to take my example, under GNOME, when my system switches to dark style automatically for the night, qalculate-gtk remains in light style.

GTK3 + libhandy, GTK4 + libadwaita or a recent version of Qt manage directly the freedesktop dark style preference, but it's possible to manually access the settings portal and fetch the preference via DBus: https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Settings.html.