checkbox-touch: add common dialog component

This patch adds CbtDialog component. This item shows popup on the screen, that
has label and set of buttons. The number of buttons is determined by the caller.

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