Handle transient notifications (#541)

[grmat]

Draft implementation for #541, implements handling of the "transient" hint according to the Desktop Notifications Specification.

The -e/--transient flag of notify-send (libnotify) allows a simple test:

notify-send -ea mailer -i email "from <[email protected]>" "Subject"

While users are still able to explicitly put notifications in the history when calling makoctl dismiss (without --no-history) or using the according binding (MAKO_BINDING_DISMISS instead of MAKO_BINDING_DISMISS_NO_HISTORY).

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Second commit is some kind of a clean-up, as it doesn't make much sense, IMHO to maintain both, a transient member and a (no-)history parameter.

[emersion]

Hm, the way I've read the spec, the "transient" bit meant that the notification is not stored on disk (and restored), but history sounded unrelated.