Checkpoint.

Expandable UI implemented.  When testing with a linear plot (for ease of review),
it appears that zooming and scaling does not adversely affect the calculated results.
Fix error in auto-addition of file extension.
Add image panning and color-identification.

Image panning only useful if image is small enough to fit entirely within the
QGraphcisView.

Text of color-selection buttons is set to be of the color selected for that
quantity.
Add dialog box to show credits for icon sources.
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.
Fix placement of markers on subsequent image loads.

Reset scaling of image view to nominal when loading a new file.  This
keeps the marker-placement consistent.
Add icon for application.
Update README.md
Add DOxygen file info comments.
Add DOxygen comments.