Add ability to print selection

Add ability to print selection

Add a new radio button "Selection" to the print dialog. Its presence
depends on calling of functions gtk_print_operation_set_support_selection()
and gtk_print_dialog_unix_set_support_selection().
Sensitivity of the radio depends on calling of
functions gtk_print_operation_set_has_selection() and
gtk_print_dialog_unix_set_has_selection().
There are new properties GtkPrintUnixDialog::support-selection,
GtkPrintUnixDialog::has-selection, GtkPrintOperation::support-selection
and GtkPrintOperation::has-selection. Corresponding getters are
gtk_print_dialog_unix_get_support_selection(),
gtk_print_dialog_unix_get_has_selection(),
gtk_print_operation_get_support_selection() and
gtk_print_operation_get_has_selection().
Application has to set number of pages to which the selection will be formated
in GtkPrintOperation::begin-print's callback by the
gtk_print_operation_set_n_pages() function (bug #344519).
There is also new property GtkPrintUnixDialog::manual-capabilities controled by
gtk_print_unix_dialog_set_manual_capabilities() and
gtk_print_unix_dialog_get_manual_capabilities().