Add custom y-axes to Analyze Statistics plot.

In the past, the y-axis of the statistics plots in Analyze has always been tied to RA and DEC plots, i.e. typically a range of -2 to 5 arcseconds. Other statistics are plotted, but the y-axis doesn't show their ranges. There also were controls for panning and zooming the y-axis, but again they only applied to RA & DEC error, along with a few other statistics with similar ranges (RMS error, Guide Drift, etc).

In this commit, a UI is introduced that allows any statistic to be used for the y-axis. Similarly, while it is tied the y-axis, it's y-axis range can be panned and zoomed. Also other attributes of the y-axis can be changed for any statistic--it's range, whether the range is computed automatically, and the plot color. The UI is described in the changes to the handbook in the commit, but, in short, you click on the statistic's QLineEdit (which displays the statistic value) and that associates the statistic with the y-axis. A double click or right-click would bring up a menu to make the changes described above.