app: add app.ID exposed to the platform.

Allow app ID to be set by linger flag -X gioui.org/app.ID=%s so that wayland
can group windows, search for ${gioui.org/app.ID}.desktop file and display
application name. e.g. /usr/share/applications/${gioui.org/app.ID}.desktop
~/.local/share/applications/${appID}.desktop.

ID is set by the gogio tool or manually with the -X linker flag.

Signed-off-by: Marko Kungla <marko.kungla@gmail.com>

app/app.go

@@ -4,6 +4,7 @@ package app
 
 import (
 	"os"
+	"path/filepath"
 	"strings"
 )
 
@@ -14,11 +15,29 @@ import (
 // Set with the go linker flag -X.
 var extraArgs string
 
+// ID is the app id exposed to the platform.
+//
+// On Android ID is the package property of AndroidManifest.xml,
+// on iOS ID is the CFBundleIdentifier of the app Info.plist,
+// on Wayland it is the toplevel app_id,
+// on X11 it is the X11 XClassHint
+//
+// ID is set by the gogio tool or manually with the -X linker flag. For example,
+//
+//	go build -ldflags="-X 'gioui.org/app.ID=org.gioui.example.Kitchen'" .
+//
+// Note that ID is treated as a constant, and that changing it at runtime
+// is not supported. Default value of ID is filepath.Base(os.Args[0]).
+var ID = ""
+
 func init() {
 	if extraArgs != "" {
 		args := strings.Split(extraArgs, "|")
 		os.Args = append(os.Args, args...)
 	}
+	if ID == "" {
+		ID = filepath.Base(os.Args[0])
+	}
 }
 
 // DataDir returns a path to use for application-specific