Resolve "Simple ncurse-alike cli to manage gosdn"

Closes #28 (closed)

cmd/gosdn-tview/app/app.go

+package app
+
+import "github.com/rivo/tview"
+
+type view interface {
+	GetContent() tview.Primitive
+}
+
+type App struct {
+	app   *tview.Application
+	pages *tview.Pages
+}
+
+func NewApp() *App {
+	a := &App{
+		app: tview.NewApplication(),
+	}
+	return a
+}
+
+func (a *App) SetRoot(v view) {
+	a.pages = v.GetContent().(*tview.Pages)
+	a.app.SetRoot(a.pages, true)
+}
+
+func (a *App) SwitchPage(s string) {
+	if a.pages.HasPage(s) {
+		a.pages.SwitchToPage(s)
+	}
+}
+
+func (a *App) AddPage(name string, p view) {
+	a.pages.AddPage(name, p.GetContent(), true, false)
+}
+
+func (a *App) Run() error {
+	return a.app.Run()
+}
+
+func (a *App) Stop() {
+	a.app.Stop()
+}
+
+func (a *App) Draw() {
+	a.app.Draw()
+}
+
+func (a *App) SetFocus(v tview.Primitive) {
+	a.app.SetFocus(v)
+}