Fixed asserts when loading toolbars

VS2012 added a new assert when loading toolbar bitmaps is they were >
8-bit; fixed by loading the legacy one by default, then replacing it
with the modern one later.
Experimental drag/drop page reordering.

-- UI bits to allow the drag/drop operations and give on-screen
feedback.
-- New messages and actions to propagate the changes through all the
observers, and actually reorder the various data structures.
-- Comprehensive sweep through the code to replace "int" page numbers
with DisplayPageNumber and RealPageNumber classes.  These are simple
wrappers for an int, but using them exclusively allows the compiler to
tell us when we make a mistake.

TODO:
-- Can't save the changes yet.
-- Haven't really tested annotations, printing, find-dialog, etc.