Consolidate input completion code in preparation for advanced completion strategies (#4639)

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

src/controllers/completion/CompletionModel.hpp

@@ -0,0 +1,44 @@
+#pragma once
+
+#include "widgets/listview/GenericListModel.hpp"
+
+#include <QObject>
+#include <QString>
+
+namespace chatterino {
+
+namespace completion {
+    class Source;
+}  // namespace completion
+
+/// @brief Represents the kind of completion occurring
+enum class CompletionKind {
+    Emote,
+    User,
+};
+
+/// @brief CompletionModel is a GenericListModel intended to provide completion
+/// suggestions to an InputCompletionPopup. The popup can determine the appropriate
+/// source based on the current input and the user's preferences.
+class CompletionModel final : public GenericListModel
+{
+public:
+    explicit CompletionModel(QObject *parent);
+
+    /// @brief Sets the Source for subsequent queries
+    /// @param source Source to use
+    void setSource(std::unique_ptr<completion::Source> source);
+
+    /// @return Whether the model has a source set
+    bool hasSource() const;
+
+    /// @brief Updates the model based on the completion query
+    /// @param query Completion query
+    /// @param maxCount Maximum number of results. Zero indicates unlimited.
+    void updateResults(const QString &query, size_t maxCount = 0);
+
+private:
+    std::unique_ptr<completion::Source> source_{};
+};
+
+};  // namespace chatterino