Migrate notifier from portmaster-ui to cmds/notifier, remove some duplicated code, move assets to assets/data and add a small go package in assets to allow embedding icons

cmds/notifier/icons.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"sync"
+
+	icons "github.com/safing/portmaster/assets"
+)
+
+var (
+	appIconEnsureOnce sync.Once
+	appIconPath       string
+)
+
+func ensureAppIcon() (location string, err error) {
+	appIconEnsureOnce.Do(func() {
+		if appIconPath == "" {
+			appIconPath = filepath.Join(dataDir, "exec", "portmaster.png")
+		}
+		err = os.WriteFile(appIconPath, icons.PNG, 0o0644)
+	})
+
+	return appIconPath, err
+}