Changed the default of output_filename_format to "[input_filename_base].gcode"

for FDM, and "[input_filename_base].sl1" for the SLA technology.
This improves the situation of
"Filename creation with variables broken"
https://github.com/prusa3d/PrusaSlicer/issues/1918
where anything after the last dot in the file name is considered
as an extension, therefore the .gcode resp. .sl1 extension is not added.
Now the user gets the output_filename_format filled in with the correct
extension.
Also now if the output_filename_format is empty, the output name
is generated anyway.

src/libslic3r/PrintBase.cpp

@@ -57,7 +57,9 @@ std::string PrintBase::output_filename(const std::string &format, const std::str
     PlaceholderParser::update_timestamp(cfg);
     this->update_object_placeholders(cfg);
     try {
-        boost::filesystem::path filename = this->placeholder_parser().process(format, 0, &cfg);
+		boost::filesystem::path filename = format.empty() ?
+			cfg.opt_string("input_filename_base") + "." + default_ext :
+			this->placeholder_parser().process(format, 0, &cfg);
 		if (filename.extension().empty())
         	filename = boost::filesystem::change_extension(filename, default_ext);
         return filename.string();

src/libslic3r/PrintConfig.cpp

@@ -1323,7 +1323,7 @@ void PrintConfigDef::init_fff_params()
                    "[input_filename_base].");
     def->full_width = true;
     def->mode = comExpert;
-    def->set_default_value(new ConfigOptionString("[input_filename_base]"));
+    def->set_default_value(new ConfigOptionString("[input_filename_base].gcode"));
 
     def = this->add("overhangs", coBool);
     def->label = L("Detect bridging perimeters");

src/slic3r/GUI/PresetBundle.cpp

@@ -78,6 +78,7 @@ PresetBundle::PresetBundle() :
     this->sla_materials.default_preset().inherits();
 
     this->sla_prints.default_preset().config.optptr("sla_print_settings_id", true);
+    this->sla_prints.default_preset().config.opt_string("output_filename_format", true) = "[input_filename_base].sl1";
     this->sla_prints.default_preset().compatible_printers_condition();
     this->sla_prints.default_preset().inherits();