frontend: warn about the dangers of massive texture packs

[xTVaser]

Fixes #310

I don't really want to do too much in regards to this because it's a problem caused by third-party stuff. So I'm just adding a warning to hopefully reduce the little frequency of this that we get.

Support wise:

• the launcher settings shows if they are using texture packs or not
• the installation logs from the tooling show if it's massive or not (it's usually obvious if they have thousands of texture replacement logs)