Add a listenner on the image exporter ui

Add a listenner on the image exporter ui to update image ratio according the selected combobox item.

Note : It seems that the current validation inputs don't work  :

inputPanel.addValidation(new MIPValidationInteger(WIDTH_T, I18N.tr("Width (pixels)")));
        inputPanel.addValidation(new MIPValidationInteger(HEIGHT_T, I18N.tr("Height (pixels)")));
        inputPanel.addValidation(new MIPValidationInteger(DPI_T, I18N.tr("DPI")));  

... @nicolas-f do you have an idea ?