Debug memory leaks.

Valgrind shows no leaks.

Cannot place markers on image, though, so this is a checkpoint commit
rather than a completion commit.
Fix loading of new image file.

Can now place markers on a freshly-loaded image (e.g., the 2nd, 3rd, etc).

Updated the project include path to point to an update location for my
general-use C++ classes.  This is now relative to the PlotExtract
project directory.