Add ext-foreign-toplevel-list protocol

This protocol provides a subset of the wlr-foreign-toplevel-management protocol. In particular this protocol provides a way for privileged clients to get a handle to a toplevel surface that may exist in another process and use the handle in other extension protocols.

This protocol is intentionally minimal as further information to describe a toplevel should be implemented with extension interfaces. For example, a protocol which may manage toplevels would use the handles defined in this protocol to reference the toplevel to set the state of. Another example use case is specifying a toplevel to capture the content of with a screencopy protocol.

An extension protocol to describe the state of a toplevel is being drafted here and uses this protocol: !196

Features provided in the wlr-foreign-toplevel-management, and the proposed ext-workspace protocols will be added in a future extension protocol and will use the handle type defined in this protocol.

• Review: @emersion
• 2 implementations:
  • Client: lswt: https://git.sr.ht/~leon_plickat/lswt/tree/toplevel-info
  • Server: cosmic: https://github.com/pop-os/cosmic-comp/pull/76
• 2 ACKs:
  • @emersion
  • @Drakulix

From the past revisions of the protocol, the following has been done:

• Add destructor requests to all objects
• Describe how extension protocols should interface with the base protocol
• Restrict the global to a single instance per client.
• Forbid having toplevel handles outlive the global (the handles become inert anyways)
• RFC 2119ify the protocol documentation