Add functions to return sublayer visibility.
Add Multiple Tracks Statistical analysis dialog.

This displays totals and averages for the selected tracks in a new dialog.

More then one instance of this dialog is allowed
 and it allows a choice of whether invisible items should be included in analysis.

NB This commit does not perform the actual selection of tracks to analysis.
Enable all tracks statistics analysis for all TRW Layers within an Aggregate layer.
Add statistics for all tracks within a single TRW layer.