UMVE: Initialize Qt5 compatible Sky Keying plugin, UMVE: Implement OMP n×n Kuwahara filter

This acts as the initial commit for UMVE's **Sky Keying** plugin. It grants a Qt5 compatible GUI (TODO: *resolve QtIcon issues for fallback icons*) and implements templated [Kuwahara filters] (http://en.wikipedia.org/wiki/Kuwahara_filter "Wikipedia article about Kuwahara filters") for any square kernel and three-channel images sped up with a few lines of OpenMP (approx. 50% performance gain).