Update libwnck3 to 3.4.9

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Version 3.4.9
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  Fixes

  * TaskList: remove new Startup Notification API

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Version 3.4.8
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  Features

  * Add support for _GTK_FRAME_EXTENTS (Rico Tzschichholz)

  Fixes

  * WindowActionMenu: unset window and stop async events on dispose
    (Marco Trevisan)
  * TaskList: use new Startup Notification API (Stefan Sperling)

  Translators

  * Fran Dieguez (gl)
  * GunChleoc (gd)
  * Milo Casagrande (it)
  * Noriko Mizumoto (ja)
  * Rudolfs Mazurs (lv)
  * Wylmer Wang (zh_CN)

(prlw1)