Fix "Keep window size for new notebook pages" to not set minimum size

Turns out I used the wrong function to keep the size of notebook pages
that were ripped off. Using gtk_widget_set_size_request on the new
notebook creates a hard minimum size for this window.

Instead we should use gtk_window_set_default_size on the new window that
is the parent of the notebook. This has the desired effect of creating the
new window with the same size as the one the page was ripped off from -
without making that the minimum size for this window.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>