feat: add --import command-line option

When specified, opens the "Import from zip" dialog as soon as the main
window is shown, with the URL field prefilled with the argument given to
the option.

Closes #2998

application/MainWindow.h

@@ -57,6 +57,8 @@ public:
     void checkInstancePathForProblems();
 
     void updatesAllowedChanged(bool allowed);
+
+    void droppedURLs(QList<QUrl> urls);
 signals:
     void isClosing();
 
@@ -180,8 +182,6 @@ private slots:
      */
     void downloadUpdates(GoUpdate::Status status);
 
-    void droppedURLs(QList<QUrl> urls);
-
     void konamiTriggered();
 
     void globalSettingsClosed();