app: add code for macOS and Windows apps under 'app' (#12933)

* app: add code for macOS and Windows apps under 'app'

* app: add readme

* app: windows and linux only for now

* ci: fix ui CI validation

---------

Co-authored-by: jmorganca <jmorganca@gmail.com>

app/dialog/util.go

@@ -0,0 +1,12 @@
+//go:build windows
+
+package dialog
+
+func firstOf(args ...string) string {
+	for _, arg := range args {
+		if arg != "" {
+			return arg
+		}
+	}
+	return ""
+}