Fixes a potential crash when changing system palette with QGtkStyle
Fixes a potential crash when changing system palette with QGtkStyle

The problem was that we installed an eventfilter regardless if the
gtk symbols were defined or not. Instead we now initialize and
check for the symbols before we install the filter.

Task-number: 254342
Reviewed-by: ogoffart

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