Marble: Add saving and loading for custom bookmark icon
completed by: Mikhail Ivchenko
mentors: Dennis Nienhüser, shentey, Sanjiban Bairagya, Torsten Rahn
During a recent commit customization of bookmark icons got added to Marble's UI. However saving the information about the icon path and restoring it doesn't work properly yet.
Please implement the missing functionality so that Marble saves and restores the information about the icon path as needed.