PlacesSidebar: Fix crash with the drag_* signals.

* tool/m4/convert_gtk.m4: Remove the conversion from GFile* to
  const RefPtr<>&. These do not belong in the general conversion
  file.
* gtk/src/placessidebar.hg: Instead put it here. These conversions
  are for signals only and care must be taken to use the
  correct reference-taking (or not) for each use.
  Bug #705642