New patch allows 2d plots to use PM3D palette colors - please test

[Ethan M. <merritt@u.washington.edu>]

I have just uploaded to SourceForge a new patchset #1072284 that allows all 
plot elements, 2D or 3D, to use PM3D palette coloring options.  
Please have a look at it.

Output from a demo script is here:
    http://www.bmsc.washington.edu/people/merritt/gnuplot/demo/palette_2D.html

The code is very simple.  The generic linetype structure now contains a 
t_colorspec substructure.  This is parsed inside lp_parse() and applied 
inside term_apply_lp_properties().  These two chunks of code thus handle all 
plot elements that have an associated linetype.

Benefits:
1) plot elements can be assigned to have arbitrary colors (a frequent request)
2) these colors are terminal-independent (another frequent request)

What's still missing:
1) Pattern fill behavior is terminal-dependent. This is fixable, but probably 
not a high priority.
2) The "save" and "show" commands do not funnel printout of linetype
properties through a shared routine. This means that there is no single place 
I can change to correctly report the palette properties of all plot elements. 
This is only a problem if you want to save your session for later reloading. 
For now the work-around is to define and use explicit linestyles.
3) ???  Tell me if you find something else I missed.