SLA only mode in wizard

Activates only if PrusaSlicer.ini does NOT contains any FFF printers.
Added only_default_printers() function to activate wizard if none printers are installed.

src/libslic3r/Preset.hpp

@@ -597,6 +597,7 @@ public:
 
     const Preset*   find_system_preset_by_model_and_variant(const std::string &model_id, const std::string &variant) const;
 
+    bool            only_default_printers() const;
 private:
     PrinterPresetCollection() = default;
     PrinterPresetCollection(const PrinterPresetCollection &other) = default;