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/Traits/Parachutable.cs

@@ -29,7 +29,7 @@ namespace OpenRA.Mods.Common.Traits
 		[Desc("Image where Ground/WaterCorpseSequence is looked up.")]
 		public readonly string Image = "explosion";
 
-		[SequenceReference("Image")]
+		[SequenceReference("Image", allowNullImage: true)]
 		public readonly string GroundCorpseSequence = null;
 
 		[PaletteReference]
@@ -37,7 +37,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		public readonly string GroundImpactSound = null;
 
-		[SequenceReference("Image")]
+		[SequenceReference("Image", allowNullImage: true)]
 		public readonly string WaterCorpseSequence = null;
 
 		[PaletteReference]