The attendance systray item needs data to determine whether or not
it should be displayed, and what to display. Before this commit, it
fetched it in the standard way, in onWillStart, and returned the
fetch promise.
However, for a systray item, it's better not to wait for the
promise, such that the webclient doesn't wait for it to be mounted.
In the case of this item, it is simply not displayed until data is
fetched, and when it is displayed, it's the right most item so it
doesn't produce any flickering.
This allows to save several ms on specific screens (e.g. home menu).
Part-of: odoo/odoo#139477
Aaron Bohy
e5161c86fe