This gives the user the ability to label different account (e.g. "work",
"private") and shows this label in the account switcher. Showing the
label in more places will be done in future MRs.
The label is stored in the user's account data and thus transfers
automatically to other instances of neochat
we get the platformtheme attached property from a random user object,
but that user is destroyed when logging out. Instead use the controller
as parent since that survives the logout
Fetching the Kirigami Theme via attached properties is expensive. The
theme is also going to be the same for all users so it's enough to only
do it once.
