r.out.gdal does not browse groups

[frankie]

Groups are valid inputs for r.out.gdal, but the browsing widget in the wx interface shows regular rasters only, so one needs to write in the group name by keyboard.

[neteler]

In i.group this is solved with

grp = G_define_standard_option(G_OPT_I_GROUP);

In r.out.gdal, input can be either G_OPT_R_INPUT or G_OPT_I_GROUP. Perhaps this must be split into two input parameters?