Do not return focus if new window is shown while popup is visible

In rare case the Sparkle window can be shown while popup is visible and
Maccy would hide it upon closing. This prevents user from being able to
update Maccy.

It's tricky to reproduce:
1. Open Maccy.
2. Open Preferences.
3. Click "Check now".
4. Close Prefrences.
5. Minize the uppdate window.
6. Un-minimize the update window.
7. Popup Maccy.
8. Un-popup Maccy.
9. Observe that when Maccy hides, the update window hides too.

Fixes #371

Maccy/Maccy.swift

@@ -306,7 +306,6 @@ class Maccy: NSObject {
   // and fallback to default NSMenu behavior by enabling
   // UserDefaults.standard.avoidTakingFocus.
   private func withFocus(_ closure: @escaping () -> Void) {
-    Maccy.returnFocusToPreviousApp = extraVisibleWindows.count == 0
     KeyboardShortcuts.disable(.popup)
 
     if UserDefaults.standard.avoidTakingFocus {
@@ -317,8 +316,9 @@ class Maccy: NSObject {
       Timer.scheduledTimer(withTimeInterval: 0.04, repeats: false) { _ in
         closure()
         KeyboardShortcuts.enable(.popup)
-        if Maccy.returnFocusToPreviousApp {
+        if Maccy.returnFocusToPreviousApp && self.extraVisibleWindows.count == 0 {
           NSApp.hide(self)
+          Maccy.returnFocusToPreviousApp = true
         }
       }
     }