Support excluding code behind files from a project.

src/Main/Base/Project/ICSharpCode.SharpDevelop.addin

@@ -422,15 +422,20 @@
 				<Or>
 					<Condition name = "Ownerstate" ownerstate = "InProject"/>
 					<Condition name = "Ownerstate" ownerstate = "None"/>
+					<Condition name = "Ownerstate" ownerstate = "BehindFile"/>
 				</Or>
-				<Condition name = "Ownerstate" ownerstate = "InProject">
+				<ComplexCondition>
+					<Or>
+						<Condition name = "Ownerstate" ownerstate = "InProject"/>
+						<Condition name = "Ownerstate" ownerstate = "BehindFile"/>
+					</Or>
 					<MenuItem id    = "ExcludeFile"
 					          label = "${res:ProjectComponent.ContextMenu.ExcludeFileFromProject}"
 					          class = "ICSharpCode.SharpDevelop.Project.Commands.ExcludeFileFromProject"/>
 					<MenuItem id     = "ExecuteCustomTool"
 					          label  = "${res:ProjectComponent.ContextMenu.ExecuteCustomTool}"
 					          class  = "ICSharpCode.SharpDevelop.Project.ExecuteCustomToolCommand"/>
-				</Condition>
+				</ComplexCondition>
 				
 				<Condition name = "Ownerstate" ownerstate = "None">
 					<MenuItem id    = "IncludeFile"