Embed "Expiry" into "Folder Properties"

The original "Expire" actions (in the "Folder" menu bar menu and
folder list popup) remain present, they open the "Folder Properties"
dialogue at the appropriate tab.

As noted in comments in KMMainWidget::setupActions(), there is
still inconstency between the wording of the action and what it does.

Hooray, a seven year old wish finally implemented!


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