gdm: Keep messages in queue until we've not fully processed them

gdm: Keep messages in queue until we've not fully processed them

It can be convenient to get the currently showing message in order to
replace or remove it in case it's not needed anymore.

So simplify the message queue handling by only depending on a single
local variable (_messageQueue) and redefining hasPendingMessages
depending on its content.

Now messages are kept in queue till they are not fully processed and the
first message is always the one currently shown.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1683>