Fix saving and restoring of window position as well as size
Commit 28480aeb implemented the use of the state configuration to store window size and position for the composer and reader windows. However, to also correctly restore the window position, as noted in the referenced bug, it is necessary to clear the flag used by KMainWindow to ensure that multiple windows do not open in exactly the same position. This is normally done by KMainWindow::closeEvent() but has to be done by SecondaryWindow itself for these windows.
The main window is also changed to use the state configuration, so that its size will be correctly restored. Restoring its position works when KMail is run without a system tray icon, but with the icon the main window always seems to open at the top left of the screen. Possibly this is a result of KWin overriding the application specified window placement only in this case.