feat(compose): a signature switch on the editor bar

A QToolButton with a checkable menu at the right end of the editor bar,
where item 142 put the controls of the editor. Not registered in KeyMap:
parented to the composer like the formatting actions, so its scope is this
window.

The signature is applied through a QTextCursor rather than setPlainText(),
which destroys the undo stack, and the seeded one is cleared from that
stack for the reason the seeded quote already is: one Ctrl+Z must not wipe
content the user never typed.

A resumed draft seeds nothing. Its body already carries the signature it
was written with, and seeding again would put a second one on a message
written once.

Part of item 152.

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