Add specs to verify the behavior of PlotPanZoomStackGroup,
which is responsible for synchronizing the domain axis
between multiple sub-plots in a stacked plot view.
WTD-625.
When changes to pan-zoom bounds are made in one
subplot, this will effect the pan-zoom bounds of
other subplots. Ensure this occurs immediately.
WTD-625.
Utilize PlotPanZoomStackGroup; that is, utilize changes
such that a set of pan-zoom stacks can be treated as one,
while their domain axis synchronizes, and their range
axis does not. WTD-625.
Initially implement a synchronized pan/zoom stack,
which maintains the same domain axis bounds for all
subplots in a stacked plot view, but allows each to
have independent range axes. WTD-625.
Place elements of the PlotController into a separate
directory inside of the source folder for the plot
plug-in, in preparation for review and integration.
Part of ongoing plot transition to Angular, WTD-533.
