checkbox-touch: defer pageStack clearing

This patch moves pageStack clearing when job is finished to when UI is ready to
display new content. This makes transition between test seem immediate (instead
of showing empty screen).

Fixes: https://bugs.launchpad.net/checkbox-touch/+bug/1392137
Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
checkbox-touch: latch everything after choice on verification

This patch makes UI to latch all buttons on verification screen once the choice
is made.

Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge of lp:~kissiel/checkbox/fix-1392137/ by tarmac [r=zkrynicki][bug=1392137][author=kissiel]"