Add high-res versions for auto-hide and unauto-hide png

1.5x is disabled for now, as Qt < 5.15.2 has a rounding bug
rendering them

src/private/widgets/TitleBarWidget_p.h

@@ -60,6 +60,8 @@ protected:
     bool isFloatButtonVisible() const override;
     bool isFloatButtonEnabled() const override;
 
+    QIcon iconForButton(const QString &iconName) const;
+
 private:
     void init();
     int buttonAreaWidth() const;