(Reserve_Space): new subprogram

This subprogram provides support for moving items aside in the Canvas_View.
This is for instance used as a special mode for overlap avoidance when users
move items interactively (see testgtk)
N724-013

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230924 936e1b1b-40f2-da11-902a-00137254ae57