desktop: don't hand an open Save dialog to another file

Some save requests open the Save dialog only after a wait: a file
opened when its download finishes, a file fetched from a linked mobile,
the database export. If a Save dialog was already open for another
file, such a request replaced the first one's result: the open dialog,
still showing the first file's name, saved the second file's content
under it, and the first request never completed.

A save request while a Save dialog is open now completes as cancelled.
The chooser is modal, so only a delayed request can arrive meanwhile.

apps/multiplatform/common/src/desktopMain/kotlin/chat/simplex/common/platform/Files.desktop.kt

@@ -75,6 +75,7 @@ actual class FileChooserLauncher actual constructor() {
       )
       res = simplexWindowState.openDialog.awaitResult(params)
     } else {
+      if (simplexWindowState.saveDialog.isAwaiting) return onResult(null)
       res = simplexWindowState.saveDialog.awaitResult(DialogParams(filename = input))
       if (res != null && res.isDirectory) {
         res = File(res, input)