feat: add model_name and plate_number placeholders (#5401)

This uses the BBS project name from the `Project` tab which could be
susceptible to upstream changes removing this feature.

The project_name template only works when you open a 3MF file. If you
create a new project and set the project name `Model.model_info` is
always null whether you save the project or not. If you save the current
project, switch to a new project/different project, then re-load it then
the template works as expected.

The plate number is assumed to always be <100 which matches the
formatting of the plate number in the UI.

Relates-To: https://github.com/SoftFever/OrcaSlicer/issues/3816

src/libslic3r/Print.hpp

@@ -903,6 +903,9 @@ public:
 
 	std::string                 output_filename(const std::string &filename_base = std::string()) const override;
 
+	std::string                 get_model_name() const;
+	std::string                 get_plate_number_formatted() const;
+
     size_t                      num_print_regions() const throw() { return m_print_regions.size(); }
     const PrintRegion&          get_print_region(size_t idx) const  { return *m_print_regions[idx]; }
     const ToolOrdering&         get_tool_ordering() const { return m_wipe_tower_data.tool_ordering; }