When I first implemented `Ctrl+G` to switch between visual mode and select mode, I re-constructed the selection for the new mode since we previously always cleared the selection when switching out of, say, visual mode.  It turns out that that the selection tracker already implements that logic for us and so we can just let the selection tracker do it job even when switching between two visual modes.

This greatly simplifies mode switching between corresponding kinds of visual modes (like character select to character visual) because the new mode just inherits the selection tracked by the selection tracker.

Without this change, a one-time switch from select to visual mode would have had to supply the visual selection in a mode argument and handle inclusive/excusive, etc. and then start the selection tracker, all with the net effect of no change in the selection.

With this change, the selection tracker does all the work as long as we don't clear the editor's selection during the mode change, whether it's a one time switch or switching via `Ctrl+G`.