Optimize: Calculate associated icon of a waypoint only on change.

Optimize: Calculate associated icon of a waypoint only on change.

Instead of looking up a hash table on every waypoint draw on every layer draw;
 store the icon reference in the waypoint itself and update only when necessary:

1. When the waypoint icon is changed directly by the user.
or
2. When implicitly changed via the size preference,
 so add method to update all windows and then all waypoints to switch between
 small<->large icons.