gtkplacessidebar.c: move bookmark at the placeholder index

As that index is set in drag_motion_callback() and visually shown
on the widget as a drop target hint.

https://bugzilla.gnome.org/show_bug.cgi?id=787356
#904
2 jobs for wip/nbenitezl/move-bookmark-placeholder-index-3 in 9 minutes (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #170437
build

00:09:00

manual #170438
win32 allowed to fail manual
msys2-mingw32