homogeneous icons

Issue generated from Tuleap's migration script. Originally submitted by: Nicolas Jager (nicolasjager-sfl)

use the same set of icons on all platforms

Google provides a lot of CC-licensed icons here: https://design.google.com/icons/\#ic\_clear

We also talked about paddings and button sizes and defined 3 sizes:
large button: 56x56, 16 padding
medium button: 40x40, 8 padding
small button: 30x30, 6 padding