Dashboard: Make dashboard update plot extent based on selected axes

Now that initial_extent is a property of the active axes object, the plot should:
- update extent (i.e. x_range/y_range) when a new axes is selected
- save currently active extent so use can switch between two sets of axes without having to re-pan/zoom to a desired extent

Start by tracing the dashboard logic for the active axes object and its initial_extent property. Check how x_range and y_range are managed when axes are selected. Done means selecting axes restores that axes' extent and preserves the currently active extent when switching between axes sets.

Written by the indexing model from the issue text.