[welcome] Implement requirements checking at module level.

- Hook up the requirements checker (which only checks) to the module-and-viewstep
   method called to do the checking.

src/modules/welcome/WelcomeViewStep.cpp

@@ -131,3 +131,8 @@ WelcomeViewStep::setConfigurationMap( const QVariantMap& configurationMap )
         cDebug() << "WARNING: no valid requirements map found in welcome "
                     "module configuration.";
 }
+
+Calamares::RequirementsList WelcomeViewStep::checkRequirements()
+{
+    return m_requirementsChecker->checkRequirements( m_widget );
+}

src/modules/welcome/WelcomeViewStep.h

@@ -57,6 +57,8 @@ public:
 
     void setConfigurationMap( const QVariantMap& configurationMap ) override;
 
+    Calamares::RequirementsList checkRequirements() override;
+
 private:
     WelcomePage* m_widget;
     RequirementsChecker* m_requirementsChecker;