Revise the bitmap Tab and related data Model, so a single IO element can have multiple bitmap coordinates

[mennowo]

Sometimes, it is needed for an IO element to be associated with more than one coordinate in the bitmap. To enable this, changes are need:

• data Model: a List<> of Point instances instead of the current X/Y ints
• ViewModels: load and save the list of points
• Bitmap Tab: enable multiple coordinates for single IO element

[mennowo]

Done. Altered the Model, and related ViewModel; no changes were needed in the View.