Add Gio app shells and platform stubs (T-003)

app-win7/platform/windows/platform_windows.go

@@ -0,0 +1,17 @@
+//go:build windows
+
+package windows
+
+type platform struct{}
+
+func newPlatform() Platform {
+	return platform{}
+}
+
+func (platform) OS() string {
+	return "windows"
+}
+
+func (platform) Edition() Edition {
+	return EditionLegacy
+}