Fix session restore
Since https://commits.kde.org/kxmlgui/8c9fb02a1d37672b26a03a9dd9e8675743deb269 KXmlGui only triggers a session restore for a window when the session config contains a "NumberOfWindows" key with a value >= 1. This commit adds such a key to the Dolphin session config, which fixes the Dolphin main window restore for KXmlGui versions containing that commit.
(cherry picked from commit c0bf226a)