app: fix warnings and crashes in rectangle select and crop
We can't rely on g_object_unref() in halt() for breaking all property GBindings between the tool options and GimpToolRectangle, because we might be in the middle of a signal emission which also refs and keeps the rectangle alive until the last callback returns. So we had dangling rectangles interacting with tool options. Remember all bindings in a list, and break them explicitly when we shut down the rectangle in halt(). Also, forgot to unset the display's highlight in the rectangle selection tool.
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