On macOS, the "drag-end" signal does not seem to be emitted in some
cases, which leaves dockables in an unsensitive state. Rather than
trying to fix GTK+2 which is half-maintained nowadays anyway, let's drop
the unsensitivity. As a comment notes, this was anyway only a visual
cue. Dragging dockables in GIMP 2.10 already shows a lot of other visual
cues, so this is redundant.
In case of drop failure, we also had to cleanup the icon widget and
detach the dockable. This can be handled by the "drag-failed" (which
hopefully work better on macOS).

As an additional improvement, I raise the newly created dockable window,
since this is the most likely behavior when you just detached a dock
(and I realize it usually appeared behind other GIMP windows, at least
on GNOME).

Note: this is a gimp-2-10 only fix as master relies on GtkNotebook DnD
code which (hopefully, as I haven't tested) doesn't have this issue. The
detached window behind other windows issue doesn't exist on master, at
the very least.