Partial support for Unicode in wgnuplot

[Bastian M. <bma...@we...>]

While gnuplot on Windows handles UTF-8 encoded files just fine, it 
currently does not support Unicode input via the keyboard. The patch at

   https://sourceforge.net/p/gnuplot/patches/727/

aims to overcome that restriction.  It makes wgnuplot (not console mode 
gnuplot) accept Unicode keyboard input.  Internally, it is converted to 
whatever the currently `set encoding` is, which may be utf8.

The display code still wrongly assumes that all characters have equal 
width, which affects command line editing. Also only code points  U+0000 
to U+D7FF and U+E000 to U+FFFF are supported.

For testing purposes I have uploaded a 64bit binary build as zip package to:

   https://sourceforge.net/projects/gnuplot/files/gnuplot/testing/

Comments greatly appreciated!
Happy testing,

   Bastian