Save and restore selection, along with cursor position when switching from node to node about leointeg

[edreamleo]

@boltex When switching nodes, Leo's core saves the insert point, selection, and vertical scroll position in the old (unselected) vnode. From v.init:

self.insertSpot = None
    # Location of previous insert point.
self.scrollBarSpot = None
    # Previous value of scrollbar position.
self.selectionLength = 0
    # The length of the selected body text.
self.selectionStart = 0
        # The start of the selected body text.