Added an checkbox option "View PDF after Export"  in Export as Pdf  Dialog Box .

If a user ticks the checkbox , pdf file will be opened after export
else only export will be done .

Goto File > Export as PDF >> Tick the checkbox "View PDF after Export "
to observe changes .

There is a lot of space in between last checkbox and this ViewPDF checkbox , because if we
open impress and check export as pdf , there are more options in the dialog.

If user export PDF and check view PDF after export, the PDF file is added to recent documents of the OS . ( tested on opensuse)

It will require make dev-install to work properly .

Change-Id: I76611c0e3382e27289d648942a98b092edc430ae
Reviewed-on: https://gerrit.libreoffice.org/3948Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>