Gui: [skip ci] replace hardcoded Alt+F4 with QKeySequence::Quit to use the appropriate shortcut for each platform

src/Gui/CommandDoc.cpp

@@ -959,7 +959,7 @@ StdCmdQuit::StdCmdQuit()
   sWhatsThis    = "Std_Quit";
   sStatusTip    = QT_TR_NOOP("Quits the application");
   sPixmap       = "application-exit";
-  sAccel        = "Alt+F4";
+  sAccel        = keySequenceToAccel(QKeySequence::Quit);
   eType         = NoTransaction;
 }
 
@@ -1158,7 +1158,7 @@ bool StdCmdPaste::isActive(void)
         return true;
     QClipboard* cb = QApplication::clipboard();
     const QMimeData* mime = cb->mimeData();
-    if (!mime)
+    if (!mime)
         return false;
     return getMainWindow()->canInsertFromMimeData(mime);
 }