turning a lot of includes into forward declares

src/widgets/dialogs/UserInfoPopup.hpp

@@ -1,6 +1,5 @@
 #pragma once
 
-#include "common/Channel.hpp"
 #include "widgets/BaseWindow.hpp"
 
 #include <pajlada/signals/signal.hpp>
@@ -9,6 +8,8 @@ class QCheckBox;
 
 namespace chatterino {
 
+class Channel;
+using ChannelPtr = std::shared_ptr<Channel>;
 class Label;
 
 class UserInfoPopup final : public BaseWindow