exposurses

- Make it function so that user is constrained to exposure menu
initially and must select value there.
- Once selected, the ISO menu becomes active and user must select value
there
- Finally make both shutter and aperture menus actives (switch between
them with left/right arrow keys). Whichever is selected, the other
will be calculated.

This is not very pretty:

- I don't like having both the selection_counter and menu_counter. Seems
almost redundant. Made sense before I constrained selections and
stepped through the menu's etc (and therefore you could re-select),
but now seems annoying to have to keep it only for how last two menus
work.
- I don't like the repetitive menu creation stuff. That should be DRY-ed
up. check-in: 580afe6ab4 user: base@atomicules.co.uk tags: origin/master, trunk