Use the Thread ID as the name for unknown thread

[CrossVR]

It isn't much, but this is slightly more useful than just using "Unknown".

[zeux]

I'm reluctant to merge this because I feel like a better solution is to keep both thread id & name and display both. That way you don't have to deduplicate thread names if you set them manually - you just have the thread id if you need it, always. The on-screen UI layer does this already but I think the web UI doesn't - there's potentially a simple fix for this that would just send the name in the web server in a similar format to what on-screen UI currently uses (see MicroProfileWriteThreadHeader in microprofileui.h), and wouldn't need further JS changes.

[CrossVR]

This change only affects threads that would previously be marked as "Unknown", it doesn't affect threads that have an actual name associated with them. I don't actually use the thread ID for anything, I just thought it was easier to read than three threads all called "Unknown".