No longer use the existance of an image pointer inside the button, or
the the type of a button to decide if the entire button should be
draggable, or only its icon. This was rather sneaky, undocumented
behavior. Instead make this a proper option in the public UI API, and
enable it by default in specific cases to match the previous behavior.
This at least makes the behavior known and controllable in the API, and
not something that's just "secretly" done deep down in the button
handling code. I'd argue we should just use the entire button width by
default for dragging, but that would be a bigger change.
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