Save and restore current okular session.

Rough proposal for implementation (without knowing details):
* Add a menu checkbox under "General" to support "Restore from last session"
* Closing okular with open tabs the open filenames will be stored somewhere (e.g. okularpartrc) along with the sequence
* On opening okular, the program will check for the session option. If activated, okular will open the filenames that have been stored. 
* Please take also care of the correct sequence of the opened tabs and also of files that have been deleted or are not accessible anymore. In my opinion, okular should not force the user to click error messages.

Please take those notes above as a proposal for this wish but feel free to implement it in any other way it suits KDE/Okular-Team best.