xdg-foreign-v2: Rename export and import calls

As export is a reserved keyword in C++, in order for the output
generated by wayland_scanner to compile correctly rename export to
export_toplevel and import to import_toplevel this needs a new protocol
version as is an incompatible change

[jadahl: Fix various documentation issues]

Signed-off-by: Marco Martin <notmart@gmail.com>
Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>