"Distance" options

One planned, but missing set of configurations is that of "distance" values.

Namely:
- separation between spouse boxes
- separation between generations
- separation between children boxes

The `AncesTree.TreeModel.Configuration` has `getGapBetweenLevels` and `getGapBetweenNodes`, but not a `getGapBetweenSpouses`.

The chart config dialog needs to be extended to provide user modification of those configuration options.

Need to confirm those configuration options are saved/restored.

Start with AncesTree.TreeModel.Configuration and the chart config dialog mentioned in the issue. Trace getGapBetweenLevels and getGapBetweenNodes, then inspect the existing configuration persistence; this is done when spouse, generation, and children spacing can be edited and all values are saved and restored.

Written by the indexing model from the issue text.