MSVC12 compile fixes & small plugin restructuring

* Fixed a little compile error in *plugin.cpp* of **osg-painters example** ( introduced with 9f4a257dc439e8c93b52d8248f417834c151fef4 )
* included *osgViewer::GraphicsWindowEmbedded* Header to fix incomplete type compile error
* put the plugins in seperate subfolder in examples & installs plugin runtime libs in subfolder "plugins" of bin dir (to get consistency between viewer-qt and viewer-qt-extended in which such structure was demanded)