New user interface for 5.29.x

[Robert Ancell <robert ancell gmail com>]

Hi all,

I've just pushed some major user interface changes onto the master
branch to be released in the 5.29.x series.

If you want to read about them have a look in the NEWS file but better
still please try them out with the perspective of a gcalctool 5.28
user and provide feedback.

Usual stuff applies for a big change at the start of a cycle:
- Nothing set in stone, please suggest improvements.  Say if it sucks.
- Expect lots of regressions.  Please patch or report if you can.
- Feedback about the areas you know better than I do. e.g. accessibility

Thanks!
--Robert