Issue #1924: Tool options mysteriously lock up due to DND/quartz issues.
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.
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