Use CalamaresUtils::lookupAndCall in PythonQtViewStep.

This is done to support both Pythonic and Qt style method names, i.e.
both "isNextEnabled" and "is_next_enabled" is ok in a Python module.
Also better documentation in PythonQtViewStep, and stub for
setConfigurationMap.
Finally, proper handling of PythonQtViewStep Python-facing basewidget:
with this change, a call to PQVS::widget() only triggers a widget
relayout if it's necessary, and leaves it alone otherwise.

src/libcalamaresui/viewpages/PythonQtViewStep.h

@@ -23,6 +23,9 @@
 
 #include <PythonQt.h>
 
+namespace Calamares
+{
+
 class PythonQtViewStep : public Calamares::ViewStep
 {
     Q_OBJECT
@@ -45,6 +48,8 @@ public:
 
     QList< Calamares::job_ptr > jobs() const override;
 
+    void setConfigurationMap( const QVariantMap& configurationMap ) override;
+
     QWidget* createScriptingConsole();
 
 protected:
@@ -56,4 +61,6 @@ private:
     friend class PythonQtViewStepDecorator;
 };
 
+}
+
 #endif // PYTHONQTVIEWSTEP_H