Add editor selection state to session history via a SelectionMemento, which

Add editor selection state to session history via a SelectionMemento, which
combines selection related editor properties with the current editor selection.

The related editor properties are:
mouse mode,
zoom setting,
left frame of the canvas,
y origin of the canvas.

Selection state now includes region views (storing the underlying region id)
and time.

This patch also fixes a region mute undo bug.