Remove Unnecessary Includes in Headers (#4275)

* refactor: remove unnecessary includes in headers

* fix: formatting

* chore: changelog

* fix: scrollbar

* fix: suggestions and old appbase remains

* fix: suggestion

* fix: missing Qt forward declarations

* fix: another qt include

* fix: includes for precompiled-headers=off

* Add missing `<memory>` includes

* Add missing `#pragma once`

* Fix tests

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/widgets/dialogs/SelectChannelDialog.hpp

@@ -1,7 +1,5 @@
 #pragma once
 
-#include "Application.hpp"
-#include "common/Channel.hpp"
 #include "widgets/BaseWindow.hpp"
 
 #include <pajlada/signals/signal.hpp>
@@ -13,6 +11,9 @@ namespace chatterino {
 
 class Notebook;
 class EditableModelView;
+class IndirectChannel;
+class Channel;
+using ChannelPtr = std::shared_ptr<Channel>;
 
 class SelectChannelDialog final : public BaseWindow
 {