Rearrange collab titlebar items to avoid movement of the toggle contacts button

* Replace username in titelbar with a `...` user menu that shows
  the current user name and contains a sign-in/sign-out button.
* Move the '+' (toggle contacts) button back to the right side.
* Move the collaborators back to the right side.
* Move the share/unshare button to the left side, beside the project title
* Only show the share/unshare button when in a call.

crates/theme/src/theme.rs

@@ -88,6 +88,7 @@ pub struct Titlebar {
     pub share_button: Interactive<ContainedText>,
     pub call_control: Interactive<IconButton>,
     pub toggle_contacts_button: Interactive<IconButton>,
+    pub user_menu_button: Interactive<IconButton>,
     pub toggle_contacts_badge: ContainerStyle,
 }