checkbox-touch: make LatchButton expose info about its state

Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
checkbox-touch: add unlatching of buttons

This patch makes Checkbox Touch unlatch buttons on current page when advancing
to the next page.

Fixes: https://bugs.launchpad.net/checkbox-touch/+bug/1392547
Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge of lp:~kissiel/checkbox/fix-1392547/ by tarmac [r=zkrynicki][bug=1392547][author=kissiel]"