Paint directly onto QWidget

In QVimShell, open a QPainter directly on the QWidget itself, rather
than painting to a QPixmap and copying onto the QWidget.

* Painting directly to the QWidget is almost always faster (according to
  the Qt graphics people). Since Qt 4, painting to QWidgets is already
  double-buffered anyway. (Issue # 78)
* Painting to a QPixmap discards subpixel rendering information when
  using the raster graphics system (the default beginning in Qt 4.8). In
  particular, font rendering quality can be substantially degraded.
  (Issue #64)