Purpose:
This pull request aims to provide an implementation of an axis line that snaps to a defined list of positions on either the horizontal or vertical axis.
The implementation tries to reuse as much code as possible while guaranteeing the proper function of the new snap mechanic for axis lines.

The general idea was to provide a feature for draggable axis lines similar to what the DraggableMarkerPlotInVector offers for draggable markers.

As this is my first pull request, I hope I correctly followed what is mentioned in CONTRIBUTING.md.
Minus the creation of an issue before starting the implementation... I started the implementation as a test balloon to get familiar with the code base and more or less because I just wanted to try it out. As it was very easy to implement, I was done faster than I first anticipated and then I got around to reading CONTRIBUTING.md but was already done...

I hope you like the new plottable. I'm looking forward to your feedback! :-)

var yPositions = DataGen.Consecutive(11, 0.2D, -1D);
var hLineWithSnap = plt.AddHorizontalLineWithSnap(0.8D, yPositions);
hLineWithSnap.DragEnabled = true;
hLineWithSnap.DragLimitMin = -1;
hLineWithSnap.DragLimitMax = 1;

var xPositions = DataGen.Consecutive(11, 5D);
var vLineWithSnap = plt.AddVerticalLineWithSnap(25, xPositions);
vLineWithSnap.DragEnabled = true;
vLineWithSnap.DragLimitMin = 0;
vLineWithSnap.DragLimitMax = 50;