Untie bonus maps and commentary buttons from game directory

[vrad-exe]

The commentary and bonus maps buttons in the new game dialog are currently tied to hardcoded game directory strings, meaning they can never show up in mods. With this now being open source it's less of a concern because modders can edit the code to make the buttons always appear, but it'd still be nice to have the ability to enable them without having to compile a custom DLL.

It'd probably make the most sense to have it controlled by 2 booleans in schemechapterbutton.res, since that file is already loaded by the new game dialog.

[DasMaddi]

I can see the use-cases for this, however, in order to best preserve backwards compatibility with HL2/Portal/Episodic, I would want to have the scheme checks override the directory check, instead of the scheme entry being the only on-off switch, if that makes sense. @Joshua-Ashton any thoughts?

[vrad-exe]

Yeah that's fine, the important thing is just giving mods a way to enable the buttons. The bonus maps button should probably also be #ifdef'd to only appear if the bonus maps menu is actually compiled in.