Gui: Disable use of qassistant

Displaying of help pages is now handled by the Help addon.
In case the Help addon is not installed, an info message is shown.

src/Base/Interpreter.cpp

@@ -468,7 +468,7 @@ void InterpreterSingleton::runFile(const char*pxFileName, bool local)
     }
 }
 
-bool InterpreterSingleton::loadModule(const char* psModName)
+bool InterpreterSingleton::loadModule(const char* psModName, bool nofail)
 {
     // buffer acrobatics
     //PyBuf ModName(psModName);
@@ -478,10 +478,14 @@ bool InterpreterSingleton::loadModule(const char* psModName)
     module = PP_Load_Module(psModName);
 
     if (!module) {
-        if (PyErr_ExceptionMatches(PyExc_SystemExit))
-            throw SystemExitException();
-        else
-            throw PyException();
+        if (nofail)
+            return false;
+        else {
+            if (PyErr_ExceptionMatches(PyExc_SystemExit))
+                throw SystemExitException();
+            else
+                throw PyException();
+        }
     }
 
     return true;

src/Base/Interpreter.h

@@ -237,7 +237,7 @@ public:
     //@{
     /* Loads a module
      */
-    bool loadModule(const char* psModName);
+    bool loadModule(const char* psModName, bool nofail=false);
     /// Add an additional python path
     void addPythonPath(const char* Path);
     static void addType(PyTypeObject* Type,PyObject* Module, const char * Name);

src/Gui/MainWindow.cpp

@@ -692,18 +692,18 @@ void MainWindow::whatsThis()
 
 void MainWindow::showDocumentation(const QString& help)
 {
-    // temporary - allows to enable/disable the use of the Help module
-    ParameterGrp::handle hGrp = App::GetApplication().GetParameterGroupByPath("User parameter:BaseApp/Preferences/Mod/Help");
-    bool useHelpModule = hGrp->GetBool("UseHelpModule", false);
-    if (useHelpModule) {
+    if (Base::Interpreter().loadModule("Help",true)) {
         Gui::Command::addModule(Gui::Command::Gui,"Help");
         Gui::Command::doCommand(Gui::Command::Gui,"Help.show(\"%s\")", help.toStdString().c_str());
     } else {
         QUrl url(help);
         if (url.scheme().isEmpty()) {
-            QString page;
-            page = QString::fromUtf8("%1.html").arg(help);
-            d->assistant->showDocumentation(page);
+            //QString page;
+            //page = QString::fromUtf8("%1.html").arg(help);
+            //d->assistant->showDocumentation(page);
+            QMessageBox::critical(getMainWindow(), tr("Help addon needed!"),
+            tr("The Help system of FreeCAD is now handled by the \"Help\" addon."
+                "Install it with menu Tools > Addons Manager"));
         }
         else {
             QDesktopServices::openUrl(url);