In gimp_session_info_restore() there is code to create a dialog from a
session info. GimpSessionInfo lives in the widgets module. Thus we
can't add restoration code that depends on a higher level module. In
particular, we can't add code to restore docks in an GimpImageWindow
since GimpImageWindow lives in the display module. And we need such
code to be able to restore a single-window mode session.

Since dialogs are defined in the dialogs module, it makes sense to
also have the code that restores a dialog in that module.

So, add a 'restore_func' member to GimpRestoreDialogFunc of type
GimpRestoreDialogFunc and move the code there.