Use new background setting capabilities for the extcairo device to insure a good background for the PostScript plot generated by the test_extcairo target.
Use new background setting capabilities for the extcairo device to insure a
good background for the PostScript plot generated by the test_extcairo
target.

Replace interactive test_plplotcanvas_demo and test_plplotcanvas_animation
targets with the interactive test_plplotcanvas target.

Add interactive test_pygcw target with tested file and target dependencies
for both the core build and installed examples build.

Tweak wording of message for interactive test_c_<device> targets to
make clear that it is an interactive device rather than a file device.

These recent interactive test changes work well as can be seen by running

make test_interactive 

either in the build tree (with BUILD_TEST=ON) or the installed examples tree
(using the new CMake-based build system for those examples).  However, there
are still additional interactive targets that must be implemented and added
as dependencies for the overall test_interactive target.

Whitespace changes.

svn path=/trunk/; revision=10575

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot