Add paper size combo and orientation combo to print dialog

Add paper size combo and orientation combo to print dialog

Paper size combo and orientation combo can be added by
gtk_print_operation_set_embed_page_setup_dialog() to GtkPrinUnixDialog
now. This function induce calling of
gtk_print_unix_dailog_set_embed_page_setup_dialog() after creation of
dialog. These two functions control embed-page-setup-dialog properties
in GtkPrintOperation and in GtkPrintUnixDialog.
There is also new function gtk_print_unix_dialog_get_page_setup_set()
which says whether page setup was set by user.
Selected page setup is stored as default page setup in
GtkPrintOperation.
New class is added, its name is GtkCustomPaperUnixDialog. The class
manages custom sizes. It is derived from GtkPageSetupUnixDialog's
CustomPaperDialog structure.
Page layout preview is modified, so, it shows dimensions of current
page setup (mm or inch - depends on locale). It also shows the name of
actual paper if page setup dialog is not embedded (paper size combo is
not visible).
gtk-demo is actualized to include this new feature.