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.
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.
Merge pull request #5769 from Mailaender/actor-lint

Added a new OpenRA.Lint check for invalid actor definitions
StyleCop
document for modders
new trait Parachtuable that decides on the fate upon landing
new setting to allow unlucky paratroopers
@pchote's quest against legacy truth value definitions
merge ParachuteAttachment and Parachutable
disallow idle animations while in the air paradropping

fixes #2988
made the parachute animation optional

instead of hard-coding default sprite sequences
add a customizable parachute shadow

closes #3335