Make StatusNotifierItem official and allow fixes

[Konrad Materka]

Hi,

The StatusNotifierItem specification 
(https://www.freedesktop.org/wiki/Specifications/StatusNotifierItem/) is 
now widely used. In fact, it is the only one supported on some 
platforms. Can it be moved to the 
https://gitlab.freedesktop.org/xdg/xdg-specs repository?

In addition, some changes and fixes are required: new properties, typos 
and other minor updates.

-- 
Konrad