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/splits/InputCompletionPopup.hpp

@@ -1,6 +1,5 @@
 #pragma once
 
-#include "common/Channel.hpp"
 #include "widgets/BasePopup.hpp"
 #include "widgets/listview/GenericListModel.hpp"
 
@@ -8,6 +7,9 @@
 
 namespace chatterino {
 
+class Channel;
+using ChannelPtr = std::shared_ptr<Channel>;
+
 class GenericListView;
 
 class InputCompletionPopup : public BasePopup