Text: support pixel offsets

[swharden]

I can place text at a point on the chart using coordinates, and I can adjust its alignment by customizing the font, but I'd love to be able to shift the relative rendering by a specified number of pixels (e.g., 5px offset down and to the right). The Test plot type could have fields like PixelOffsetX and PixelOffsetY which default to 0.

Those fields would simply get added to these pixel values here:

ScottPlot/src/ScottPlot/Plottable/Text.cs

Lines 89 to 90 in 9b4aaa2

	float pixelX = dims.GetPixelX(X);
	float pixelY = dims.GetPixelY(Y);

[bclehmann]

I could see this being useful for annotations, arrows, and tooltips as well?