Allow mapping half axes to buttons

This allows to bind pressure sensitive NeGcon buttons to keyboard,
mouse and controller buttons

src/duckstation-qt/qtutils.h

@@ -1,9 +1,12 @@
 #pragma once
 #include <QtCore/QByteArray>
+#include <QtCore/QMetaType>
 #include <QtCore/QString>
 #include <initializer_list>
 #include <optional>
 
+Q_DECLARE_METATYPE(std::optional<bool>);
+
 class ByteStream;
 
 class QFrame;