citra-qt: Use structured bindings where applicable in ConfigureHotkeys::applyConfiguration. Fix a few minor issues.

* Address https://github.com/citra-emu/citra/pull/4437#discussion_r241380912

src/citra_qt/configuration/configure_dialog.h

@@ -17,7 +17,7 @@ class ConfigureDialog : public QDialog {
     Q_OBJECT
 
 public:
-    explicit ConfigureDialog(QWidget* parent, const HotkeyRegistry& registry);
+    explicit ConfigureDialog(QWidget* parent, HotkeyRegistry& registry);
     ~ConfigureDialog() override;
 
     void UpdateVisibleTabs();