Add GtkFlowBox

Add GtkFlowBox

GtkFlowBox is a container that its children in a reflowing
grid, which can be oriented horizontally or vertically.

It is similar to GtkListBox in that the children can
be sorted and filtered, and by requiring a dedicated child
widget type, GtkFlowBoxChild. It is similar to GtkTreeView
in that is supports a full set of selection modes, including
rubberband selection.

This is the culmination of work that has happened in the
egg-list-box module, and earlier in libegg. The origins of
this code are the EggSpreadTable in libegg, which was written
by Tristan van Berkom. It was moved to egg-list-box and
renamed EggFlowBox by Jon McCann, and I gave it some finishing
touched in the flowbox-improvements branch of that module.