ui/app: create windows directly

Replace CreateWindow with NewWindow that immediately creates a Window
ready to use.

Drop the Windows channel of windows created by the system. For iOS
and Android where the system creates the windows, let them rendezvous
with the window created in the first NewWindow call.

Android is further changed so that destroying and re-creating the
Java Activity simply reconnects with the original Window.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/os_windows.go

@@ -3,6 +3,7 @@
 package app
 
 import (
+	"errors"
 	"fmt"
 	"image"
 	"runtime"
@@ -159,11 +160,11 @@ func Main() {
 	<-mainDone
 }
 
-func createWindow(opts *WindowOptions) error {
+func createWindow(window *Window, opts *WindowOptions) error {
 	onceMu.Lock()
 	defer onceMu.Unlock()
 	if len(winMap) > 0 {
-		panic("multiple windows are not supported")
+		return errors.New("multiple windows are not supported")
 	}
 	cerr := make(chan error)
 	go func() {
@@ -176,14 +177,14 @@ func createWindow(opts *WindowOptions) error {
 		}
 		defer w.destroy()
 		cerr <- nil
-		windows <- w.w
+		w.w = window
+		w.w.setDriver(w)
 		showWindow(w.hwnd, _SW_SHOWDEFAULT)
 		setForegroundWindow(w.hwnd)
 		setFocus(w.hwnd)
 		if err := w.loop(); err != nil {
 			panic(err)
 		}
-		close(windows)
 		close(mainDone)
 	}()
 	return <-cerr
@@ -248,7 +249,6 @@ func createNativeWindow(opts *WindowOptions) (*window, error) {
 	if err != nil {
 		return nil, err
 	}
-	w.w = newWindow(w)
 	return w, nil
 }