Split `platform::unix` into `platform::x11` and `platform::wayland`

[madsmtm]

There were basically no overlap between the platform-specific extensions in these, so it made little sense to keep them in the same file!

Will gladly rename the functions as well, e.g. wayland_set_csd_theme -> set_csd_theme?

~Blocked-ish on #2420~

[notgull]

I agree with this decision. Wayland and X11 are very different, so it makes sense to keep them in their own separate modules.