BUG: Shift-MouseMove with Crosshairs (cross-referencing) would blink crosshair

When using Crosshair in cross-referencing mode (as opposed to navigator mode) and
using Shift-MouseMove to jump the slices, the crosshair on the other slices would
sometimes disappear or blink.

This is caused by the crosshair being turned off when it is not on the current slice.

The crosshair was not always on the current slice because the CrosshairDisplayableManager
could receive the MouseMove event BEFORE the InteractorStyle (which handles the jump slice
behavior). Hence the crosshair was built for one slice but since the slicenode had not been
updated, it wasn't on the slice being display (yet) and was turned off.

Set the priority of the MouseMoveEvent to -1 for the CrosshairDisplayableManager so that
the InteractorStyle receives the event first, which allows the slice to jump to the proper
RAS before the crosshair is drawn.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21187 3bd1e089-480b-0410-8dfb-8563597acbee