Rewrite sequence linting / missing file handling.

- Distinguish between missing sequences and missing sprites
- Lint default sequences as well as maps
- Improved performance
- Correctly handle null images

OpenRA.Mods.Common/Projectiles/Bullet.cs

@@ -36,7 +36,7 @@ namespace OpenRA.Mods.Common.Projectiles
 		[Desc("Image to display.")]
 		public readonly string Image = null;
 
-		[SequenceReference("Image")]
+		[SequenceReference("Image", allowNullImage: true)]
 		[Desc("Loop a randomly chosen sequence of Image from this list while this projectile is moving.")]
 		public readonly string[] Sequences = { "idle" };
 
@@ -57,7 +57,7 @@ namespace OpenRA.Mods.Common.Projectiles
 		[Desc("Trail animation.")]
 		public readonly string TrailImage = null;
 
-		[SequenceReference("TrailImage")]
+		[SequenceReference("TrailImage", allowNullImage: true)]
 		[Desc("Loop a randomly chosen sequence of TrailImage from this list while this projectile is moving.")]
 		public readonly string[] TrailSequences = { "idle" };