fix mac specific button modification on addressbookpage

- continue the mac behavior of clearing button icons (because it's unusual on mac apps)
- fix: new button variable names, new buttons (verifyMessage, signMessage)

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge pull request #2498 from jonasschnelli/addressbookpage_mac_fix

fix mac specific button modification on addressbookpage