Blender Git Loki

Add new GridFlow layout.

This mimics the 'spreadsheet' behavior. Columns and/or rows can have
equal sizes, or adapt to their content - but always in a grid way (i.e.
all items in a same column will always have same available width, and
all items in a same row will always have same available height).
Also, you can fill (order) the grid in a row- or column-major way.

Not used anywhere for now.

Differential: https://developer.blender.org/D2395