Fixes to the plot view UI: will make the UI conform to PlotUI.txt

-Make the mouse target area ~20% (5 pixels) of the size of the mouse marker.
 The mouse will be pointing at the highest view object which is anywhere 
 inside the target area, to make it easier to select thin items, like thin
 lines.

-View objects which are children of 2dplots need to be positioned relative to the
 axis box, not relative to the whole kst2dplot, so that if, eg, the axis label font
 size is changed (changing the border with) the child view object will stay fixed 
 relative to the axis (imagine an arrow pointing to a feature).

 The solution to this appears to be to place and draw children relative to the
 parent's contentsRect, rather than their geomentry().  Setting the viewport to
 contentsRect() rather than geometry() in kstviewobject::paint() does this, drawing
 the child relative to the contents rect (set, for 2dplot, to the axis box).  
 But there are problems with this:
 	-the clip regions are still relative to geometry
	-drag and drop are still relative to geometry
	-The child can't be moved outside the contents rect.
 Hmmm....

More bugs:
- Lines and arrows are clipped
- Selection points are always clipped even if the object lies below.

