Set _q_readonly property for online-help (#2890)

- Setting _q_readonly disables the timestamp check of the registered
  online-help.qch file
- This enables displaying of Contents and Index of the online help,
  even if the timestamp of online-help.qch is slightly different.
  Differences in the timestamp can result from packing online-help.qch
  in a cab file, when building the installer (1 or 2 seconds) or from
  installing the file under a different timezone under Linux etc.
- Remark: Qt 5.14 or newer is required to display Contents or Index,
  when the help files are readonly

gui/helpdialog.cpp

@@ -66,6 +66,8 @@ HelpDialog::HelpDialog(QWidget *parent) :
     }
 
     mHelpEngine = new QHelpEngine(helpFile);
+    // Disable the timestamp check of online-help.qhc by setting _q_readonly
+    mHelpEngine->setProperty("_q_readonly", QVariant::fromValue<bool>(true));
     mHelpEngine->setupData();
 
     mUi->contents->addWidget(mHelpEngine->contentWidget());