UI: Add free function to set background of widget transparent

src/UI/Utils.cc

+#include "Utils.hh"
+#include <QWidget>
+
+using namespace Vivy;
+
+void
+Utils::setTransparentBackgroundForWidget(QWidget *const w) noexcept
+{
+    w->setAttribute(Qt::WA_NoSystemBackground);
+    w->setAttribute(Qt::WA_TranslucentBackground);
+    w->setAttribute(Qt::WA_TransparentForMouseEvents);
+}

src/UI/Utils.hh

+#pragma once
+
+#ifndef __cplusplus
+#error "This is a C++ header"
+#endif
+
+class QWidget;
+
+namespace Vivy::Utils
+{
+void setTransparentBackgroundForWidget(QWidget *const) noexcept;
+}