BUG: Fixed various errors in the save scene dialog for mrb format

There were 3 problems:
1. When the save scene dialog is shown first, the file format selector is empty
2. When a scene is saved as .mrb and then the save dialog is opened again the format is still .mrb, but the populated items are not disabled
3. When a scene is saved as .mrb and then the save dialog is opened again the target directory is not the same (it’s the parent directory of the directory where the scene was saved the last time)