Tab to next editable widget

[cmyr]

When in a single-line text field or other editable widget, tab should move to the 'next' widget.

A challenge with this problem is that while it's easy enough to move 'forward' (down-tree, continuing from the top until you reach your starting point) it's harder to move up the tree.

In any case I think I have a reasonably simple solution: we build a 'responder chain' (this is a term borrowed from Cocoa that refers to a pattern that handles this behaviour as well as a bunch of other stuff) on startup or when widgets are added and removed, and then moving to the previous or next widget is just a matter of walking forwards and backwards in this chain; so we have to do some work up front (basically sending a RegisterResponder life cycle event, where widgets that want to be involved in tab navigation could register their interest) and then everything at navigation time is peachy.

This work will almost certainly be blocked on #432 and #404.

[cmyr]

closed by #482