BT: Fix a painting glitch in gtk combo box
BT: Fix a painting glitch in gtk combo box

This improves the look of combo box with gtk style which is somewhat a
regression from 4.5.0 since we did not style this part before.

Reviewed-by: nrc
(cherry picked from commit f2edb497ac2c3ff6f8c004046e216b8bdf7c4347)

KDE/qt,KDE/qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,RLovelett/qt,radekp/qt,RLovelett/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,radekp/qt,RLovelett/qt,radekp/qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,radekp/qt,KDE/qt,radekp/qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,radekp/qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,radekp/qt,KDE/qt,radekp/qt,radekp/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,radekp/qt,igor-sfdc/qt-wk,RLovelett/qt,igor-sfdc/qt-wk,RLovelett/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,KDE/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,KDE/qt,KDE/qt,igor-sfdc/qt-wk,KDE/qt,RLovelett/qt,RLovelett/qt