Add one() handler to bus & bex

[robert-hoffmann]

I know i can probably handle double bindings/triggers myself, but it's repetitive, and in bex mode you have all kinds of weird problems that seem to pop up, and this should probably handle it.

So a one() handler that makes sure nothing triggers twice would be nice 🙏

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I've also noticed some oddities with how bexContent(), bexBackground() is loaded between pages, content, and background.
It sometimes gets tricky to coordinate an event system between them, because of sometimes random load order.
(i'm probably gonna end up having them call each other, to be sure to get the hooks up ..but also need to prevent double hooks)

I'll post more on that when i have time to make a blank sample project and do some traces...