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/wintray/winclass.go

@@ -0,0 +1,45 @@
+//go:build windows
+
+package wintray
+
+import (
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// Contains window class information.
+// It is used with the RegisterClassEx and GetClassInfoEx functions.
+// https://msdn.microsoft.com/en-us/library/ms633577.aspx
+type wndClassEx struct {
+	Size, Style                        uint32
+	WndProc                            uintptr
+	ClsExtra, WndExtra                 int32
+	Instance, Icon, Cursor, Background windows.Handle
+	MenuName, ClassName                *uint16
+	IconSm                             windows.Handle
+}
+
+// Registers a window class for subsequent use in calls to the CreateWindow or CreateWindowEx function.
+// https://msdn.microsoft.com/en-us/library/ms633587.aspx
+func (w *wndClassEx) register() error {
+	w.Size = uint32(unsafe.Sizeof(*w))
+	res, _, err := pRegisterClass.Call(uintptr(unsafe.Pointer(w)))
+	if res == 0 {
+		return err
+	}
+	return nil
+}
+
+// Unregisters a window class, freeing the memory required for the class.
+// https://msdn.microsoft.com/en-us/library/ms644899.aspx
+func (w *wndClassEx) unregister() error {
+	res, _, err := pUnregisterClass.Call(
+		uintptr(unsafe.Pointer(w.ClassName)),
+		uintptr(w.Instance),
+	)
+	if res == 0 {
+		return err
+	}
+	return nil
+}