This attempts to solve bug
[#714921](https://bugzilla.gnome.org/show_bug.cgi?id=714921).
They are available as two buttons on the format bar, next to font
options. The icons I used are taken from a free icons site just for demo
purposes and should be replaced by new icons. Also I did not include
hotkeys mainly because I could not come up with a good one, also they
are pretty uncommon I think.
Also added new icons to icons/CMakeLists.txt
