Support newlines in GUI description field

[HebaruSan]

Problem

It was noted in #2796 that line breaks in module descriptions work on Mono but not Windows:

Cause

Mono uses "\n" to split lines because it runs on Unix.
Windows uses "\r\n" to split lines because CP/M did in 1974.
The TextBox implementations carry this difference through to 2019.

Changes

Now we will replace both \r\n and \n with Environment.Newline, which contains the string used by the current platform to break lines. This way authors may include either \r\n or \n in descriptions and it will always display correctly.

Note that this is not a fix for #2796 because that issue also requests bold and italic.

[DasSkelett]

I take the freedom to approve this PR without testing it (on Windows) this time.
In case you have doubts that it works on other people's machines too, you will have to wait a few days ;-)