Fix Dolphin session management regression
The commit fff7573ebb910712ad97951bf1762e6a7bb0bdc7 introduced a
regression concerning restoration of saved Dolphin sessions. The
main window numbers which can be passed to KMainWindow::restore()
start with 1, and not with 0.
CCBUG:275649
(cherry picked from commit b4091809beedf0e373ac2f497634cc0bedf77a8f)