Allow cancellation of HTTP requests

`NETWORK::HttpRequest()` now returns a HttpRequestFuture with a
`Cancel()` method.

Docs/Luadoc/Lua.xml

@@ -2051,6 +2051,9 @@
 			<Function name='GetHeld'/>
 			<Function name='GetActive'/>
 		</Class>
+		<Class name='HttpRequestFuture'>
+			<Function name='Cancel'/>
+		</Class>
 		<Class base='ActorFrame' name='TextBanner'>
 			<Function name='Load'/>
 			<Function name='SetFromSong'/>
@@ -2498,6 +2501,7 @@
 			<EnumValue name='&apos;HttpErrorCode_TooManyRedirects&apos;' value='13'/>
 			<EnumValue name='&apos;HttpErrorCode_ChunkReadError&apos;' value='14'/>
 			<EnumValue name='&apos;HttpErrorCode_CannotReadBody&apos;' value='15'/>
+			<EnumValue name='&apos;HttpErrorCode_Cancelled&apos;' value='16'/>
 		</Enum>
 		<Enum name='ImageCacheMode'>
 			<EnumValue name='&apos;ImageCacheMode_Off&apos;' value='0'/>

Docs/Luadoc/LuaDocumentation.xml

@@ -3560,7 +3560,7 @@ end
 	<Function name='IsUrlAllowed' return='bool' arguments='string url'>
 		Returns true if access to <code>url</code> is allowed.
 	</Function>
-	<Function name='HttpRequest' return='void' arguments='table params'>
+	<Function name='HttpRequest' return='HttpRequestFuture' arguments='table params'>
 		Performs an HTTP request.<br />
 		Usage example:
 <pre><code>
@@ -6210,6 +6210,12 @@ local spr = Def.Sprite{
 		Returns true if the note was initiated.
 	</Function>
 </Class>
+<Class name='HttpRequestFuture'>
+	<Function name='Cancel' return='void' arguments=''>
+                Cancels the running HTTP request. Does nothing if the request
+                has already completed.
+	</Function>
+</Class>
 <Class name='TextBanner' grouping='Actor'>
 	<Function name='Load' return='void' arguments='string sMetricsGroup'>
 		Loads the TextBanner from the specified metrics group.