fix Gdal Merge nodata value and UI

this pull request has 2 commits

- fix nodata value of the GDalTools merge widget

This essentially adds the -a_nodata argument when nodata is set

Discussed in this thread
http://osgeo-org.1560.x6.nabble.com/No-data-value-in-Raster-Merge-not-working-td5091670.html

- put the creation options in a collapsible group box, as in other widgets