Reinstate gowid Screen's support for activation and deactivation

We recently merged changes to help with a gowid use case where an
application wanted to provide its own tcell Screen object and not use
the one gowid creates. In doing so, I inadvertently broke correct screen
activation and deactivation. Activation means that the terminal screen
is taken over by the tcell screen, and deactivation means control is
given back and in practice the terminal screen's contents will return to
their appearance before the screen was activated. For example, in
termshark, if the user listens on an interface, termshark's UI will not
start right away, displaying:

(The termshark UI will start when packets are detected on eth0...)

It behaves like this in case listening on an interface requires
superuser permissions, which tshark will request - so this lets the user
see that prompt without the UI covering it up.

When packets arrive, termshark activates the tcell screen and the UI
appears. If the user hits ctrl-z, termshark deactivates the screen and
sends itself a SIGSTOP. This lets the user see what was on the terminal
before termshark launched. Emacs in the terminal behaves the same way
and I copied that.

I lost this behavior with recent gowid updates and didn't spot it until
recently. The fix here is to reinstate the DontActivate bool to the
App's options, but to only make use of it if the user does not supply
their own screen. If the gowid app passes its own tcell screen to the
gowid App, then ActivateScreen and DeactivateScreen should not be used.

app.go

@@ -99,7 +99,8 @@ type App struct {
 	prevWasMouseMove     bool // True if we last processed simple mouse movement. We can optimize on slow
 	enableMouseMotion    bool
 	enableBracketedPaste bool
-	// systems by discarding subsequent mouse movement events.
+	screenInited         bool
+	dontOwnScreen        bool
 
 	lastMouse    MouseState    // So I can tell if a button was previously clicked
 	MouseState                 // Track which mouse buttons are currently down
@@ -117,6 +118,7 @@ type AppArgs struct {
 	EnableMouseMotion    bool
 	EnableBracketedPaste bool
 	Log                  log.StdLogger
+	DontActivate         bool
 }
 
 // IUnhandledInput is used as a handler for application user input that is not handled by any
@@ -283,9 +285,10 @@ func newApp(args AppArgs) (rapp *App, rerr error) {
 		log:                  args.Log,
 		enableMouseMotion:    args.EnableMouseMotion,
 		enableBracketedPaste: args.EnableBracketedPaste,
+		dontOwnScreen:        args.Screen != nil,
 	}
 
-	if args.Screen == nil {
+	if !res.dontOwnScreen && !args.DontActivate {
 		if err := res.initScreen(); err != nil {
 			return nil, err
 		}
@@ -809,14 +812,13 @@ func (a *App) Quit() {
 // I can't make tcell claim and release the same screen successfully. Clients of
 // the app struct shouldn't cache the screen object returned via GetScreen().
 //
+// Assumes we own the screen...
 func (a *App) ActivateScreen() error {
 	screen, err := tcellScreen()
 	if err != nil {
 		return WithKVs(err, map[string]interface{}{"TERM": os.Getenv("TERM")})
 	}
-	if a.screen != nil {
-		a.screen.Fini()
-	}
+	a.DeactivateScreen()
 	a.screen = screen
 	if err := a.initScreen(); err != nil {
 		return err
@@ -827,16 +829,21 @@ func (a *App) ActivateScreen() error {
 	return nil
 }
 
+// Assumes we own the screen
 func (a *App) DeactivateScreen() {
-	a.screen.Fini()
-	a.screen = nil
+	if a.screen != nil && a.screenInited {
+		a.screen.Fini()
+		a.screen = nil
+		a.screenInited = false
+	}
 }
 
 func (a *App) initScreen() error {
 	if err := a.screen.Init(); err != nil {
 		return WithKVs(err, map[string]interface{}{"TERM": os.Getenv("TERM")})
 	}
 
+	a.screenInited = true
 	a.initColorMode()
 
 	defFg := ColorDefault