Add simple dialog to show DEM file information.

Includes source of DEM file data and filename+timestamp of the file
on disk.

Based on the methods used in the dialog to show map tile information.
Scope to include additional menu items for an Aggregate Layer.
Correct tooltip message for paste behaviour
Enable changing name of an existing waypoint in the Waypoint Properties dialog.
Fix waypoints may not be shown if drag copied between layers.

Need to recalculate waypoint bounds in all drag circumstances.