app,internal/window: extract native window code to separate package

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

app/runmain.go

@@ -1,27 +0,0 @@
-// +build android darwin,ios
-
-package app
-
-// Android only supports non-Java programs as c-shared libraries.
-// Unfortunately, Go does not run a program's main function in
-// library mode. To make Gio programs simpler and uniform, we'll
-// link to the main function here and call it from Java.
-
-import (
-	"sync"
-	_ "unsafe" // for go:linkname
-)
-
-//go:linkname mainMain main.main
-func mainMain()
-
-var runMainOnce sync.Once
-
-func runMain() {
-	runMainOnce.Do(func() {
-		// Indirect call, since the linker does not know the address of main when
-		// laying down this package.
-		fn := mainMain
-		go fn()
-	})
-}