extras: optional sdk download resource list

[unitycoder]

would need list of tools/packages/resources download links list (like android sdk/jdk/ndk),
list should be some easy-to-fetch over web (without updating launcher).

this is to save time from searching correct packages/downloads.

[unitycoder]

this could be just a 1 button link to github md (with those infos), no need to load anything from web

[unitycoder]

tools tab now has Resources link button to
https://github.com/unitycoder/UnityResources *content still to be filled