notification: escape notification message body

Notification daemons generally support markup in the message, and
when chatty tries to send a notification with text that fails markup
parsing then the notification does not contain the text at all.

E.g. receiving a SMS with the contents: "<wooohoo>" generates the
following warning in Phosh:

(phosh:22722): Gtk-WARNING **: 13:57:15.935: Failed to set text '<wooohoo>' from markup due to error parsing markup: Error on line 1 char 24: Element “markup” was closed, but the currently open element is “wooohoo”

I also took the liberty to reorder the switch cases to reduce duplicate
code.
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