Enable odd pages left option evert time dual page is enabled.
There are two states to keep track of: the document model and
the menu. The document model is updated every time. We can force
updating the menus every time the value of dual page changes.

Fixes #602

According to the PDF spec, the creation date and modification date are
optional (for pdf documents). We can safely assume that a file will not
to be modified exactly at 31 Dec 1969 23:59:59 UTC, so each creation and
modification date equal to -1 is considered as NULL for the pointers to
the other attributes (None is displayed).

The if statements could be used in the future when using GDateDime instead
of GTime (a pointer could help, or optional type), and it works for
any type of document, not only PDF.

Closes: #1549

Signed-off-by: Philippe Volte--Vieira <philippeisvv@hotmail.fr>