- use same set of icons than in other clients
- use predefined sizes for buttons and icon padding
   large button: 56x56, 16 padding
   medium button: 40x40, 8 padding
   small button: 30x30, 6 padding

Tuleap: #354
Change-Id: Ic3a01665e666419ae10a9f36955669149d8e5c01