Add support for QT term in gnuplot

This patch inserts 'qt' as terminal type for gnuplot for all OSes, with lower
precedence than 'wxt' but higher than 'x11'

Qt is a better alternative than x11, according to
[gnuplot 4.6.0 release notes](http://www.gnuplot.info/announce.4.6.0):

```
        NOTES TO PACKAGERS AND TESTERS
       ===============================

Configuration options for interactive use
-----------------------------------------

The 4.6 source code supports three primary cross-platform output modes in
addition to several platform-specific modes.

1) Cairo/pango/wxWidgets
  These terminals were introduced in version 4.4 and are now the most
  stable and full-featured option.  This set of terminals includes
  - pngcairo, pdfcairo, epscairo, and cairolatex for output to a file
  - wxt for interactive display
  This is the default configuration, but requires prior installation of
  libcairo, libpango, libcairo, libwxgtk, and related support libraries
  To disable these terminals:
      ./configure --disable-wxt --without-cairo

2) Qt
  The new qt terminal supports interactive display with menu-driven
  output to png, svg or pdf.
  Requires libqt version >= 4.5
      ./configure --enable-qt

3) X11 (the "classic" interactive interface)
  This used to be the preferred interactive interface, but the newer
  wxt and qt terminals offer nicer output and a wider range of features.
```