Adapt ManhattanStyle for QCheckBox in toolbar

Since https://codereview.qt-project.org/c/qt-creator/qt-creator/+/282887 a checkbox is displayed in a Qt Creator tool bar: Debug Mode > Clang Tidy & Clazy

Qt Creator's custom style does not seem to handle check boxes in the tool bar and as a result, the check box looks pretty alien as is:

In particular,

1. The check box does not look disabled in the same way as the QToolbutton next to it
2. The check box (rectangle + text) is painted some pixels above the base line of the QToolButton next to it