added skia backend for WRasterImage
fixed layout problem causing overlapping widgets
Several updates:

    - fix #2478: deleting a widget bound in a WTemplate not cleaning up properly
    - fix #2488: tstui2 case, bad propagation of preferred size through tabwidget
    - handle touch-events in WGLWidget; WRasterImage fix
    - WToolBar::addButton(WSplitButton) has AlignmentFlag
    - fix repeated headers regression (#2501)
    - WToolBar::addButton
    - add touch support to WSlider
    - fix #2328: do not select an item without contents
    - implemented server-side rendering (fallback) for WGLWidget, added 3DCharting library
fixes for GL server-side rendering on Windows