app: [API] remove Main

All platforms already allow the omission of the call to Main and running
Windows on the main goroutine. This change just gets rid of Main, and
documents the special requirement on Window.Event.

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

app/os_windows.go

@@ -85,10 +85,6 @@ var resources struct {
 	cursor syscall.Handle
 }
 
-func osMain() {
-	select {}
-}
-
 func newWindow(win *callbacks, options []Option) {
 	done := make(chan struct{})
 	go func() {