(Set_Visibility_Threshold, Show, Hide): new subprograms

Add support for controlling the visibility of items in a canvas_view.
Part of N718-017

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230758 936e1b1b-40f2-da11-902a-00137254ae57
Add support for hiding or showing links in a canvas_view.

Part of N718-015

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230761 936e1b1b-40f2-da11-902a-00137254ae57
Fix node positioning in graph layout.

The following graph was not properly aligned:

    A --> B --> C
            \-> D
            \-> E

instead of

            /-> C
    A --> B --> D
            \-> E

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230776 936e1b1b-40f2-da11-902a-00137254ae57
Similar fix to the previous one, for node ordering.

When looking at parent nodes, we should go from the highest layer down
to the lower layer, which makes the algorithm more efficient.

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230814 936e1b1b-40f2-da11-902a-00137254ae57
Canvas_View layout: add support for showing dummy vertices

This is mostly useful for debugging the layout algorithm by making
those normally invisible vertices visible. This modifies the display
of the links as well, since long edges are replaced by short straight
ones.

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230815 936e1b1b-40f2-da11-902a-00137254ae57
Change previous checkin, by using waypoints for long edges

This replaces the actual insertion of the dummy vertices, and means that
we are correctly highlighting the related items in GPS when we selected
an item.

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230820 936e1b1b-40f2-da11-902a-00137254ae57
Disable debug information

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230821 936e1b1b-40f2-da11-902a-00137254ae57
Layered layout algorithm can now use waypoints

This is no longer just a debug option, but is an explicit parameter that
users can chose to use.
To make this more usable interactively, the waypoints are removed when
we start moving items, for the links that will also move.

git-svn-id: http://svn.eu.adacore.com/anonsvn/Dev/trunk/GtkAda@230824 936e1b1b-40f2-da11-902a-00137254ae57