protocols/workspace: Set ext workspace id for pinned workspace

The `id` is defined to be sent only once, on creation of the handle or
later. And only for workspaces that are "likely to be stable across
multiple sessions".

Set we add an `id` initially for pinned workspaces, and add one when the
workspace is pinned.

The `id` is not supposed to be human readable, so we just use a random
value.

src/shell/mod.rs

@@ -400,8 +400,7 @@ fn create_workspace_from_pinned(
             } else {
                 TilingState::FloatingOnly
             },
-            // TODO Set id for persistent workspaces
-            None,
+            pinned.id.clone(),
         )
         .unwrap();
     state.add_workspace_state(&workspace_handle, WState::Pinned);