Patched multiple instances recent files bug
Multiple instances of okular now have a coherent history of recent files.
The method writeSettings() (invoked by the destructor) of the object Shell used to read and write the history of recently opened files as well as other settings, so calling the destructor used to overwrite any change possibly made to the history by other instances in the meantime.
The problem has been solved by creating specific methods for writing and reading the recent files history, and by making the destructor update the recent files history before writing the settings file.