turning a lot of includes into forward declares

src/widgets/AttachedWindow.hpp

@@ -1,12 +1,14 @@
 #pragma once
 
+#include <QTimer>
 #include <QWidget>
 
-#include "common/Channel.hpp"
-#include "widgets/splits/Split.hpp"
-
 namespace chatterino {
 
+class Split;
+class Channel;
+using ChannelPtr = std::shared_ptr<Channel>;
+
 class AttachedWindow : public QWidget
 {
     AttachedWindow(void *_target, int _yOffset);