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_windows.go

@@ -157,7 +157,7 @@ const _WM_REDRAW = _WM_USER + 0
 var onceMu sync.Mutex
 var mainDone = make(chan struct{})
 
-func Main() {
+func main() {
 	<-mainDone
 }