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/util/sequence_dialog/sequence_dialog.h

@@ -0,0 +1,24 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <memory>
+#include <QDialog>
+#include <QKeySequenceEdit>
+
+class SequenceDialog : public QDialog {
+    Q_OBJECT
+
+public:
+    explicit SequenceDialog(QWidget* parent = nullptr);
+    ~SequenceDialog();
+
+    QKeySequence GetSequence();
+    void closeEvent(QCloseEvent*) override;
+
+private:
+    QKeySequenceEdit* key_sequence;
+    bool focusNextPrevChild(bool next) override;
+};