Fix file extensions persisting in file dialog after export

This caused issues if one decided to export many formats in a row.
The new file extension would be appended to the previous one.

Now, the filename is retained without its extension for successive exports.

Fixes #7291

editor/project_export.h

@@ -99,6 +99,8 @@ private:
 	Label *export_error;
 	HBoxContainer *export_templates_error;
 
+	String default_filename;
+
 	void _patch_selected(const String &p_path);
 	void _patch_deleted();