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/MultiMC.h

@@ -6,6 +6,7 @@
 #include <QFlag>
 #include <QIcon>
 #include <QDateTime>
+#include <QUrl>
 #include <updater/GoUpdate.h>
 
 #include <BaseInstance.h>
@@ -221,5 +222,6 @@ private:
 public:
     QString m_instanceIdToLaunch;
     bool m_liveCheck = false;
+    QUrl m_zipToImport;
     std::unique_ptr<QFile> logFile;
 };