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.cpp

@@ -0,0 +1,35 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <QDialogButtonBox>
+#include <QVBoxLayout>
+#include "citra_qt/util/sequence_dialog/sequence_dialog.h"
+
+SequenceDialog::SequenceDialog(QWidget* parent) : QDialog(parent) {
+    setWindowTitle(tr("Enter a hotkey"));
+    auto* layout = new QVBoxLayout(this);
+    key_sequence = new QKeySequenceEdit;
+    layout->addWidget(key_sequence);
+    auto* buttons =
+        new QDialogButtonBox(QDialogButtonBox::Ok | QDialogButtonBox::Cancel, Qt::Horizontal);
+    buttons->setCenterButtons(true);
+    layout->addWidget(buttons);
+    connect(buttons, &QDialogButtonBox::accepted, this, &QDialog::accept);
+    connect(buttons, &QDialogButtonBox::rejected, this, &QDialog::reject);
+    setWindowFlags(windowFlags() & ~Qt::WindowContextHelpButtonHint);
+}
+
+SequenceDialog::~SequenceDialog() = default;
+
+QKeySequence SequenceDialog::GetSequence() {
+    return QKeySequence(key_sequence->keySequence()[0]);
+}
+
+bool SequenceDialog::focusNextPrevChild(bool next) {
+    return false;
+}
+
+void SequenceDialog::closeEvent(QCloseEvent*) {
+    reject();
+}

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;
+};