Add username autocompletion popup menu (#2866)

src/widgets/splits/InputCompletionItem.hpp

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <functional>
+#include "messages/Emote.hpp"
+#include "widgets/listview/GenericListItem.hpp"
+
+namespace chatterino {
+
+class InputCompletionItem : public GenericListItem
+{
+    using ActionCallback = std::function<void(const QString &)>;
+
+public:
+    InputCompletionItem(const EmotePtr &emote, const QString &text,
+                        ActionCallback action);
+
+    // GenericListItem interface
+public:
+    virtual void action() override;
+    virtual void paint(QPainter *painter, const QRect &rect) const override;
+    virtual QSize sizeHint(const QRect &rect) const override;
+
+private:
+    EmotePtr emote_;
+    QString text_;
+    ActionCallback action_;
+};
+
+}  // namespace chatterino