use cairocffi instead of pycairo

As suggested by @brutasse and for #825
Merge pull request #920 from esc/use_cairocffi_backport_0.9.x

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 #923 from obfuscurity/backport_new_toolbar_icons

Backport new composer toolbar icons to 0.9.x