If you go to Preferences > User Interface / Options and enable the "Autosave Desktop" option, all previously opened files and objects will reopened when starting PL/SQL Developer.
It will not reload unsaved changes though. I have added this to the list of enhancement requests.
new version of ultraedit, the "session option" function very nice, love it.
Hope pl/sql developer havs the same function as far as possible, 'session option' to save all state is very nice, realy very nice func.