rose edit: alter latent variable layout

This alters the layout of variables on a page. Latent ('ghost') variables are no longer sorted to the bottom, but intermixed with the 'real' ones. This means that the page structure is relatively preserved when adding and removing variables.

All variables for a page are now constructed at the same time, but latent ones are hidden. The old behaviour was to add them if the show latent variable mode was on or was toggled on.

This can be tested by going to an app and deleting variables, adding blank or from latent variables, and switching the `View Latent Variables` menu option on and off.

@arjclark, please review.