rename icons in accordance with Icon Naming Specification

See http://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html
prefer to use icons from theme, return icon of all the available sizes as fallback
don't use icon size in IconManager, don't update actions' icons

IconManager returns icons that support all available sizes.
So we don't need to set icon size in it and we don't need to update
actions, because they can get pixmap of required size automatically.
Merge pull request #56 from volkov0aa/master

Prefer to use icons from system icon theme