Refactor ConcurrentMap

* Add operator[] to ConcurrentMap which returns a TValue reference
* BTTV/FFZ channel emotes are now stored in the Emote Manager, and each Channel object has a reference to their own BTTV/FFZ channel emote map.
* Restructure EmoteManager a bit (simplify the ConcurrentMap havoc).
* Add EmoteData struct which can store emote data (for now only messages::LazyLoadedImage*)
* Add CompletionManager that does nothing

src/completionmanager.hpp

@@ -0,0 +1,51 @@
+#pragma once
+
+#include <QAbstractItemModel>
+
+#include <string>
+
+namespace chatterino {
+
+class CompletionModel : public QAbstractItemModel
+{
+public:
+    virtual int columnCount(const QModelIndex & /*parent*/) const override
+    {
+        return 1;
+    }
+
+    virtual QVariant data(const QModelIndex &index, int role) const override
+    {
+        // TODO: Implement
+        return QVariant();
+    }
+
+    virtual QModelIndex index(int row, int column, const QModelIndex &parent) const override
+    {
+        // TODO: Implement
+        return QModelIndex();
+    }
+
+    virtual QModelIndex parent(const QModelIndex &child) const override
+    {
+        return QModelIndex();
+    }
+
+    virtual int rowCount(const QModelIndex &parent) const override
+    {
+        return 1;
+    }
+};
+
+class CompletionManager
+{
+    CompletionManager();
+
+public:
+    CompletionModel *createModel(const std::string &channelName);
+    void updateModel(CompletionModel *model, const std::string &channelName);
+
+    friend class Application;
+};
+
+}  // namespace chatterino