Add batching for mod downloads

[SamuelMereau]

This will group mods in batches of 5 and attempt to download all of them. If the mod is downloaded, it will remove it from the batch and add another mod which will then begin downloading.

[iliadsh]

Very neat, though I tried this change and it seems like the mods are only downloaded one at a time, so the other bars just idle. Did it work with your Steam?

Also just personal preference, but I feel like 5 is too many at least for the current UI :p