Add focus tests to test-flow-box
Work on focus handling

Consistently skip insensitive items.
Rename get_average_item size

It computes a maximum, not an average, so call it
get_max_item_size.
Improve selection behaviour

Make the multi-selection keybindings a bit more conventional. In
general, Shift extends the selection, and Ctrl modifies it. As
a nod toward touch, we allow selections to be modified by single
unmodified clicks.
Add a TODO

This TODO lists things EggFlowBox needs to get ready to
become GtkFlowBox.