use cairocffi instead of pycairo

As suggested by @brutasse and for #825
Merge pull request #919 from esc/use_cairocffi

use cairocffi instead of pycairo
remove unused icons and replace toolbar icons with a unified style
new icons converted from Font Awesome project
use alternate calendar icon
Merge pull request #922 from obfuscurity/new_toolbar_icons

Replace old composer toolbar icons