game_list: Use forward declarations where applicable

Reduces the number of header dependencies propagated by the header file.

src/citra_qt/game_list.cpp

@@ -4,17 +4,27 @@
 
 #include <QApplication>
 #include <QFileInfo>
+#include <QFileSystemWatcher>
+#include <QHBoxLayout>
 #include <QHeaderView>
 #include <QKeyEvent>
+#include <QLabel>
+#include <QLineEdit>
 #include <QMenu>
+#include <QModelIndex>
+#include <QStandardItem>
+#include <QStandardItemModel>
 #include <QThreadPool>
+#include <QToolButton>
+#include <QTreeView>
+#include "citra_qt/game_list.h"
+#include "citra_qt/game_list_p.h"
+#include "citra_qt/main.h"
+#include "citra_qt/ui_settings.h"
 #include "common/common_paths.h"
 #include "common/logging/log.h"
 #include "common/string_util.h"
 #include "core/loader/loader.h"
-#include "game_list.h"
-#include "game_list_p.h"
-#include "ui_settings.h"
 
 GameList::SearchField::KeyReleaseEater::KeyReleaseEater(GameList* gamelist) {
     this->gamelist = gamelist;