Loaders: Add cancelation to all file loaders.

Mainly, for HTTP which might be stalled trying to connect (especially if
you're not near your PC right now and it's in your recent, for example.)

Core/FileLoaders/CachingFileLoader.cpp

@@ -25,7 +25,7 @@
 
 // Takes ownership of backend.
 CachingFileLoader::CachingFileLoader(FileLoader *backend)
-	: filesize_(0), backend_(backend), exists_(-1), isDirectory_(-1), aheadThread_(false) {
+	: backend_(backend) {
 }
 
 void CachingFileLoader::Prepare() {
@@ -288,3 +288,7 @@ void CachingFileLoader::StartReadAhead(s64 pos) {
 	});
 	th.detach();
 }
+
+void CachingFileLoader::Cancel() {
+	backend_->Cancel();
+}