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

staging/ext-foreign-toplevel-list/ext-foreign-toplevel-list-v1.xml

@@ -207,12 +207,15 @@
 
     <event name="identifier">
       <description summary="a stable identifier for a toplevel">
-        This identifier is used to check if two toplevel handles are the same.
+        This identifier is used to check if two or more toplevel handles belong
+        to the same toplevel.
 
         The identifier is useful for command line tools or privileged clients
         which may need to reference an exact toplevel across processes or
         instances of the ext_foreign_toplevel_list_v1 global.
 
+        The compositor must only send this event when the handle is created.
+
         Compositors must ensure the identifier is the same for all handles of the
         toplevel. The identifier is only valid as long as the toplevel is mapped.
         If the toplevel is unmapped and then remapped in the future, the identifier