ui/app: add package documentation and document Main

Split Main into the common Main function and platform specific
main functions.

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

ui/app/os_macos.go

@@ -271,7 +271,7 @@ func createWindow(win *Window, opts *WindowOptions) error {
 	return <-mainWindow.errs
 }
 
-func Main() {
+func main() {
 	wopts := <-mainWindow.out
 	view := viewFactory()
 	if view == 0 {