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

[boltex]

Save and restore selection, along with cursor position, from selection state saved in each node (or gnx array)

Addendum: Once implemented, this can be tested by trying to run the 'execute script' command on a text selection via leo's minibuffer (alt+x) instead of (ctrl+b) or the command palette (ctrl+shift+p).

Can clise this issue after removing the internal ctrl+b execute script to use leo's own command directly like the rest of normal commands.

[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.