Recognise .ttinclude file extension as a t4 template.

src/AddIns/Misc/TextTemplating/Project/Resources/TextTemplating.xshd

@@ -1,7 +1,7 @@
 <SyntaxDefinition
 	xmlns="http://icsharpcode.net/sharpdevelop/syntaxdefinition/2008"
 	name="TextTemplating"
-	extensions=".tt;.t4">
+	extensions=".tt;.t4;.ttinclude">
 	
 	<Color name="TemplateBlockStartEndTags" foreground="Black" background="Yellow" />
 	<Color name="DirectiveKeywords" foreground="Brown" fontWeight="bold" />

src/AddIns/Misc/TextTemplating/Project/TextTemplating.addin

@@ -25,7 +25,7 @@
 	<Path name="/Workspace/Icons">
 		<Icon
 			id="TextTemplateFileIcon"
-			extensions=".tt;.t4"
+			extensions=".tt;.t4;.ttinclude"
 			resource="TextTemplate.ProjectBrowser.File"/>
 	</Path>
 	
@@ -43,7 +43,7 @@
 	<Path name="/SharpDevelop/ViewContent/AvalonEdit/SyntaxModes">
 		<SyntaxMode
 			id="TextTemplating.SyntaxMode"
-			extensions=".tt;.t4"
+			extensions=".tt;.t4;.ttinclude"
 			name="TextTemplating"
 			resource="ICSharpCode.TextTemplating.Resources.TextTemplating.xshd"/>
 	</Path>