Updates to foreign-toplevel-info

This commit applies some review comments I made on wayland/wayland-protocols!155 (closed)

In particular:

• Destructor was added to ext_foreign_toplevel_info_v1
• Add error values to ext_foreign_toplevel_info_v1
• Add error for creating a second ext_foreign_toplevel_info_v1
• Add error when destroying ext_foreign_toplevel_info_v1 with existing child handles

The rationale for the defunct_toplevels error is that it makes little sense to have the toplevel handles stay valid past the manager. Also the compositor can send the "finished" event to stop the global.

• RFC 2119ify the protocol documentation
• Describe how extension protocols should interact with the protocol. e.g. foreign-toplevel-management
• Describe how to interpret the array payload in the ext_foreign_toplevel_handle_v1.state event

Signed-off-by: i509VCB git@i509.me