Defaults and language icons were missing, and graph/profile
icon was not readable.
All icons except "language.png" were taken from Google's
Material Design Icon pack which is distributed under Apache 2.0 license
(see https://github.com/google/material-design-icons)
Language icon is from http://languageicon.org site. If their
license isn't permissive enough, then we can switch to a language icon
from Material Design pack (globe).
Signed-off-by: Sergey Starosek <sergey.starosek@gmail.com>
