docs: Ignore harder

docs: Ignore harder

First of all, we must list *all* ignored headers. Since we have public
headers in the x11 and wayland directories, we must explicitly declare
all headers that we consider private under those directories.

The "quartz" subdirectory was renamed "macos", with the new macOS
backend. The "mir" directory was removed, so there's no need to ignore
it.

We are also missing a bunch of ignored headers in the top-level gdk
directory.

Finally, pass the list of ignored files to gtkdoc-mkdb, so we won't get
missing declaration warnings.