Fix Windows dialog folder titles

[Semphriss]

Description

Fix titles being copied without carrying the terminating NULL byte.

Same fix as in cf946e3, which was not done for the folder implementation.

Existing Issue(s)

Fixes #12000 (comment)

[slouken]

Can you review all the string handling in both subsystems?

[slouken]

Also, is there a reason you're not using WIN_UTF8ToStringW()?

[Semphriss]

Also, is there a reason you're not using WIN_UTF8ToStringW()?

There's one place (the filters) where it's easier to use MultiByteToWideChar, because the string that must be converted contains null bytes. For this place in particular, it's just because I didn't know about that function when I first wrote the code, and in bugfixing PRs I often go for the most straightforward fix. I'll change it to use WIN_UTF8ToStringW.

I'm sorry for the multitude of basic mistakes, I've been distracted by personal stuff lately. I'll try to review the string handling in the upcoming days, but I'll soon be unavailable for a while, so I would be more comfortable if someone else could cover for trays and dialogs in the meantime. If you wish, we can talk about it privately.

[slouken]

I'll take over on the tray and dialog work. Good luck with your personal stuff!