Gui: Fix broken toolbars layout due to MaterialWorkbench

The explicit activation of the MaterialWorkbench breaks the toolbars layout for every start. When fixing it manually it will be broken
again after the next start.

Because the core doesn't depend on the Material module it's a no-go to add an explicit runtime dependency to the corresponding workbench.

Since the Part module depends on the Materials module and the PartGui on MatGui the correct way is to let Part an PartGui load their
dependencies.

src/Gui/StartupProcess.cpp

@@ -451,9 +451,6 @@ void StartupPostProcess::showMainWindow()
 
 void StartupPostProcess::activateWorkbench()
 {
-    // Always activate the material workbench
-     guiApp.activateWorkbench("MaterialWorkbench");
-
     // Activate the correct workbench
     std::string start = App::Application::Config()["StartWorkbench"];
     Base::Console().Log("Init: Activating default workbench %s\n", start.c_str());