Various little fixes here and there:

	- Basic support for dragging the selection is in (still does
	  not do anything, will do the action binding later).

	- Fixes to the cell-selection-code-when-you-type-a-formula code

	- ExprTrees are now properly refcounted (instead of only
	  refcounting the root node).  This is required for the
	  Paste special/with-arithmetic operation feature.

	- Expression decoding (for pasting a formula with relative
	  references) now takes into account operator precedence to
	  avoid cluttering the pasted expression with extra paretheses.

	Other smallish fixes which I forget now.

Miguel.