Replace build_workspace fn with an initialize function that takes a workspace

This makes it clearer that the function is not providing necessary
dependencies to a workspace, but rather configuring it with all of
the panels and widgets which are defined in downstream crates.

crates/contacts_panel/src/contacts_panel.rs

@@ -1111,8 +1111,8 @@ mod tests {
                 client,
                 user_store: user_store.clone(),
                 fs,
-                build_window_options: || unimplemented!(),
-                build_workspace: |_, _, _| unimplemented!(),
+                build_window_options: || Default::default(),
+                initialize_workspace: |_, _, _| {},
             }),
             server,
         )