Added button to copy the current virtual tileset image to the clipboard.
Work-around for QGLPixelBuffer bug in Qt 5.1.1.

QGLPixelBuffer::format() doesn't return the correct value, which causes
QGLPixelBuffer::toImage() to return an image lacking an alpha channel.

Wrote minimal PixelBuffer class replacement based on the Qt sources.

Bug reported to qt-project.org.
Fixed "UseVirtualTilesets" setting being restored incorrectly.
Changed make-dist.tcl for Qt 5.1.1