Save default imagery layers (background image) without having to use a session

[the_redburn]

Currently, the only way to set default imagery layers is to use sessions (​https://help.openstreetmap.org/questions/37141/another-josm-set-a-default-background-image-at-start-up).

There should be an easier way to set default imagery layers.

Alternatively, there should be a way to easily save the current session as default, without having to manually save the session somewhere. This could be done automatically (without asking the user) if the user isn't using sessions.