The cursor should only be shown when there is no selection, since this
is how it is done in Windows.  This patch avoids showing the cursor when
there is a selection, and destroys the cursor when a selection is made.

Previously word drag and shift selection was not implemented.  Line
drag selection was working, but shift selection wasn't.

The error was a memory access of a freed object.  In ME_AddUndoItem I
checked the top of the undo stack to end a coalescing undo transaction,
assuming that this should be either a valid undo item, or NULL, instead
it was already freed.