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.
```
Merge pull request #170 from jucor/gnuplotqt

Add support for QT term in gnuplot