Bug 622758 - Fix error display for thumbnail creation

Do not stay forever in "Creating Preview..." state during
auto-thumbnailing, and fix error showing when creation fails.
Bug 676522 - Make DND work between images in *one* dockable...

...after in-DND image switching

This was actually two issues:

First, DND data is normally transferred on drop. In this situation the
contents of the source widget change in the middle of DND, so when the
drop happens there is nothing to transfer. Fixed this by attaching the
thing to transfer (image, item, color etc) to the GdkDragContext when
the DND operation starts, and trying to retrieve it on drop. Only when
nothing is attached we fall back to the traditional way of asking the
source widget. This is expected and an obvious fix.

Second, and not so obvious, the source part of the GTK+-internal state
of the DND operation (GtkDragSourceInfo) contains a *reference* (not a
copy) of the source widget's target list. When we change images in the
middle of DND, the source layer view's target list gets modified, and
because GtkDragSourceInfo only references it, the state of the ongoing
DND operation gets destroyed with it. Fixed this by changing
gimp_dnd_data_source_remove() to never change a source widget's target
list but instead create a new list without the removed target and
replace the source widget's list, keeping the ongoing drag's list
unaffected.

Also kept all the GIMP_LOG() stuff I added during debugging there, it
turned out to be quite useful.