Implemented naming of the SLA export file based on the output file name

template.

Reworked naming of the plater exports to not use the output file name
template, but to derive the file name from the first printable object's name.

Fixed error handling: Reimpemented the Perl's "eval" blocks
as try / catch blocks.

src/libslic3r/Model.hpp

@@ -653,6 +653,9 @@ public:
     static std::string get_auto_extruder_id_as_string(unsigned int max_extruders);
     static void reset_auto_extruder_id();
 
+    // Propose an output file name based on the first printable object's name.
+    std::string         propose_export_file_name() const;
+
 private:
     MODELBASE_DERIVED_PRIVATE_COPY_MOVE(Model)
 };