flow box

[Matthias Clasen <matthias clasen gmail com>]

I've pushed a flowbox branch, which adds a GtkFlowBox widget. It is a copy of the EggFlowBox widget that has been developed in the egg-list-box module for a while, which is in turn based on an earlier EggSpreadTable in libegg.

I think the widget is more or less ready to land in GTK+. It has
- accessibility
- height-for-width
- keynav

- documentation

- multi-selection including rubberband selection and autoscroll

- sorting and filtering

Some things could still be added:

- baseline alignment

- headers

I'd appreciate review. I'm hoping to land this in the next week or so.

Matthias