1. When adding a button to flowbox, the flowbox widget interposes a
flowboxchild between the two, which can receive events and so becomes
part of the tab order. this is why our buttons weren't getting
activated - they weren't focused even though they looked focused. So,
use labels instead of buttons for completions
2. For some reason I don't understand, flowboxchild widgets receive
:activate *only* on keyboard activation. So instead of using it,
we connect to :child-activated on the flowbox.
3. Setting widget:on_foo *adds* a handler to the widget instead of
replacing what was previously there. There is no nice way to remove
handlers from a widget either. Because we need a new
on_child_activated handler every time the completions change,
the only way I can see to achieve this is to create the flowbox
afresh on each keystroke instead of creating it once when the
frame is created
Daniel Barlow