Fixes the following problems in the Browser (described in ID 22424,
22276, 22260, 6085, 20158, 3498):

* drag from the class list with no class selected results in dragging
'nil'. Things break when this is dropped on them.

* drag from the class list to the class category list is broken.

* drag from the method list with no method selected drags
SomeClass->#Definition

* drag an uncategorized method from the method list to another empty
class in the same browser results in a syntax error

* dragging was inconsistent: in class lists if you have one class
selected and mouse down on another, the selected class is dragged, while
in method lists you drag the item you started dragging on.

This changes the policy as follows:
	* dragging requires selection
	* the dragged item is always the selection

This allows a model to answer nil as the dragPassenger; if it does, then
the drag will not be initiated.

Also fixes the inability to drag from a MessageSet (ID 8265).
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