[Kst] Checkin 482465

[Andrew Walker]

Regrettably the attempts to fix the problem with x/y zoom did not work.
I believe this is because the changes didn't actually prevent the
call to p.setClipRegion(_lastClipRegion); because the exact same
call is made at the start of Kst2DPlot::updateXYGuideline(...)
as well. Commenting out this latter call makes the XY Zoom Mode
work properly, but introduces a new problem:

Create 4 plots. Enter layout mode. Partially obscure one plot
with another and enter Y Zoom Mode. Move the mouse over the
obscured plot and notice the guideline is drawn over multiple plots.