Print centerprint messages to the console (#159)

Print centerprint messages to the console (#159)

Add new function `Con_CenterPrint` which can be used to print messages
to the console, without showing onscreen, and with a "horizontal rule"
above and below. Use `Con_CenterPrint` to log centerprint messages to
the console.

![image](https://github.com/DarkPlacesEngine/darkplaces/assets/424218/8ef0bdc4-7481-4f11-b0a3-d46e4d451c74)

Fixes #98

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Signed-off-by: bones_was_here <bones_was_here@xonotic.au>
Co-authored-by: bones_was_here <bones_was_here@xonotic.au>