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/slic3r/GUI/BackgroundSlicingProcess.hpp

@@ -55,7 +55,8 @@ public:
 	void set_finished_event(int event_id) { m_event_finished_id = event_id; }
 
 	// Activate either m_fff_print or m_sla_print.
-	void select_technology(PrinterTechnology tech);
+	// Return true if changed.
+	bool select_technology(PrinterTechnology tech);
 
 	// Get the currently active printer technology.
 	PrinterTechnology   current_printer_technology() const;