Add Qt Wayland platform plugin

1. Bug https://bugs.gentoo.org/show_bug.cgi?id=480654
"[qt overlay] dev-qt/qtgui with USE="gles2" doesn't enable opengles2
in QT_CONFIG" should be fixed before, or
/usr/lib/qt5/mkspecs/qconfig.pri edited manually.
2. qtwayland-5.9999 USE=wayland-compositor doesn't compiles right now
(upstream issue)

Signed-off-by: Maksim Melnikau <maxposedon@gmail.com>