A dialog for File->Merge

[micheus]

Well, I was working in a way to select only the desired elements from a scene when using the Merge command, but then I realise that show all the elements name in a dialog can't be done as it should be since we don't have a way to scroll the window if it is too "long" (too many elements).
So, by now, I add the option to select the categories we want to import:

Does someone think this can be useful? If so, I can pull a request for the main devs in order to try include it in the next version of Wings3d.

In case someone would like to test it here is the link for download.