Curve tool improvements

(Relates to #121)

- Fix yet more issues with curve preset save/load behavior.  This tool
is so tricky with presets, but I think I've finally worked out all the
positioning issues, and all the localization issues.  I wanted this to
be 100% perfect before adding separate RGB curves to the mix (coming
soon!).

- Added dynamic curve coordinate display to the UI.  If the mouse is
within distance of a node, the displayed position will be locked to that
node.  Otherwise, it will display the interpolated value of the curve at
the current mouse x-location.

- Added x/y guidelines to the UI, for the current mouse position.  This
should help make precision work even easier, especially if the user is
trying to align curve data to the histogram.