Log Message:
Update to 0.18:

Devil's Pie "Simmer To A Sizzle Like The Days Of Old" 0.18
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* Add window_property matcher (Nigel Tao)
* Add set_viewport action (James Willcox)
* Add opacity action (Pavel Palat)
* Add sticky action (Steve Leung)
* Add unmaximise action (Alex Menk)
* Add unminimise action (Richard Neill)
* Fix negative offsets in geometry action (Vincent Ho, Midred)