Add an "id" field to TextMenu.Item.

This allows us to search for a menu item using a numeric identifier, instead of having to keep TextMenu.Item references around everywhere.
Add a "Circularize" menu to MJ,

plus take advantage of the new TextMenu.Item.id field to get rid of a few now-unneeded TextMenu.Item references.
Update JSIOrbitDisplay to show vessel's location at encounter, even if it's on nextPatch.
Add a helper static class to access the Gizmo textures' coordinates.
Update steerable camera to use the new GizmoIcons utility class.
Add prograde vector icons to heading bar and ladder.

Provides a proxy for slip and angle of attack.
Fixed orbital displays, and smoothed out ellipse rendering (periapses should look smoother on high e orbits).
Merge pull request #73 from MOARdV/master

Surge of productivity.  A few code changes.