notebook: avoid crash on tab DND

See "gtk_notebook_detach_tab" API documentation. Using it instead
of gtk_container_remove avoids an assertion crash in gtk+ when
dragging and dropping a tab between terminal windows.

See also original bug report at
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=825818

https://bugzilla.gnome.org/show_bug.cgi?id=769161
(cherry picked from commit 85b448f7)