Fix of G-code Viewer not loading .gco files #5536

".gcode", ".gco", ".g" and ".ngc" were considered to be G-code file
extensions by the G-code import / export file dialogs, but not by
various other functions. Now the G-code extension is tested by
a single function is_gcode_file(string).

src/slic3r/GUI/Plater.cpp

@@ -4847,9 +4847,7 @@ void Plater::load_gcode()
 
 void Plater::load_gcode(const wxString& filename)
 {
-    if (filename.empty() ||
-        (!filename.Lower().EndsWith(".gcode") && !filename.Lower().EndsWith(".g")) ||
-        m_last_loaded_gcode == filename)
+    if (! is_gcode_file(into_u8(filename)) || m_last_loaded_gcode == filename)
         return;
 
     m_last_loaded_gcode = filename;