citra-qt: Add base support for hotkey reconfiguration + UI (whole of PR citra-emu/citra#3786)

* Adds a new Hotkeys tab in the Controls group.
* Right click to reconfigure.
* See the original PR for more details & screenshots.

src/citra_qt/configuration/configure_hotkeys.h

@@ -0,0 +1,48 @@
+// Copyright 2017 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <memory>
+#include <QStandardItemModel>
+#include <QWidget>
+#include "common/param_package.h"
+#include "core/settings.h"
+
+namespace Ui {
+class ConfigureHotkeys;
+}
+
+class HotkeyRegistry;
+
+class ConfigureHotkeys : public QWidget {
+    Q_OBJECT
+
+public:
+    explicit ConfigureHotkeys(QWidget* parent = nullptr);
+    ~ConfigureHotkeys();
+
+    void applyConfiguration(HotkeyRegistry& registry);
+    void retranslateUi();
+
+    void EmitHotkeysChanged();
+
+    void Populate(const HotkeyRegistry& registry);
+
+public slots:
+    void OnInputKeysChanged(QList<QKeySequence> new_key_list);
+
+signals:
+    void HotkeysChanged(QList<QKeySequence> new_key_list);
+
+private:
+    void Configure(QModelIndex index);
+    bool IsUsedKey(QKeySequence key_sequence);
+    QList<QKeySequence> GetUsedKeyList();
+
+    std::unique_ptr<Ui::ConfigureHotkeys> ui;
+
+    QList<QKeySequence> input_keys_list;
+    QStandardItemModel* model;
+};