Add mechanism to control focus inside dialogues

[demiankatz]

As discussed on #1171: sometimes when you activate a dialogue box with the keyboard, the focus does not end up in the optimal location. It may be useful to add a mechanism (e.g. a data element on controls) that specifies a preferred focus within the opened dialogue, in order to allow override if the default selection is sub-optimal.

[LlGC-jop]

Thanks @demiankatz,

I won't assign myself just yet, but if no-one else has picked this up by mid-next-week I'll probably take a look.