Remove minor keyword incompatibilities with C++

Created by: nitaigao

Hi wlroots Team 👋

When working on my compositor I have noticed there are a small number of statements that cause some incompatibilities with C++.

• A few static array statements are peppered throughout the headers.
• The xwayland struct has a member named class which is a reserved keyword.

I humbly raise this PR to make the adjustments to fix up these issues. Other than these pretty much everything else works great with C++.

Thanks for all the work on such a great framework, i hope this doesn't inconvenience too much.