Use portals by default

It can be safely assumed that a system has portals by default; if they don't, many things will simply fail to work, including, but not limited to, the dark mode preference and many other freedesktop settings.

Those that absolutely refuse to use D-Bus will thus end up with an unusable system in the first place, if they want to use anything desktop-related. So enabling the portal backend by default is fine.

GTK will, of course, fall back to using the portals if it's running inside of a Flatpak environment, but it requires manual workarounds outside of one. And, of course, having a library depend on GTK when it's more or less unnecessary isn't ideal.

Cargo.toml

@@ -11,7 +11,7 @@ repository = "https://github.com/PolyMeilex/rfd"
 documentation = "https://docs.rs/rfd"
 
 [features]
-default = ["gtk3"]
+default = ["xdg-portal"]
 file-handle-inner = []
 gtk3 = ["gtk-sys", "glib-sys", "gobject-sys"]
 xdg-portal = ["ashpd", "urlencoding", "pollster"]