(Scroll_Into_View): add support for animation and showing a rectangle

This can be used after inserting multiple items in the browser, so that as
many of them as possible become visible in the view.

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230939 936e1b1b-40f2-da11-902a-00137254ae57
(Insert_And_Layout_Items): new subprogram

Moves code from GPS, which allows "expanding" an item without having to
redo the whole layout, and thus better preserve user changes.
N724-013

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230946 936e1b1b-40f2-da11-902a-00137254ae57