placessidebar: Take a GdkDrag in _set_drop_targets_visible()

Since the function is usually called from GtkWidget::drag-{begin,end} handlers,
taking a GdkDrop does not work, especially given that
::drag-action-requested is emitted without checking the type.

Fixes #1220
4 jobs for places-sidebar-dnd in 58 minutes and 33 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Build
passed #65703
fedora-x86_64

00:12:35

passed #65704
win32
msys2-mingw32

00:08:35

 
  Flatpak
passed #65705
flatpak:demo

00:04:55

passed #65706
flatpak:widget-factory

00:45:57