Remove redundant FindActorsInBox helpers.
Add World parameter to Target.FromOrder.
Add World parameter to Target.FromCell.
Add World parameter to Util.BetweenCells.
Replace CPos.CenterPosition -> Map.CenterOfCell.
Remove WVec.ToCVec.
Replace WPos.ToCPos -> Map.CellContaining.
Remove conversions between legacy and world types.
Remove CVec -> WVec conversion.
Changes ISpriteSource.Frames to be of type IReadOnlyList<ISpriteFrame>.

- Updated implementations to return a ReadOnlyList around an array (to reduce wasted memory from exposing lists or lazy enumerators around lists).
- Protect non-public ISpriteFrame classes by making them inner classes to prevent casting.
- Added an AsReadOnly extension method for lists.
Merge pull request #5758 from pchote/cell-world-conversions

Generalize cell/world coordinate conversions.
new check for undefined actors
remove legacy actors as a test case
now that we check actors and players we can't exclude them
and another hit for invalid players
don't spam passed checks
only display lint check name when something goes wrong
tell us what you are doing
explain the strange console message
Merge pull request #5766 from RoosterDragon/read-only-frames

Make sprite frames read-only lists.