Separation of libdocument

[spixi]

libdocument is used in Evince and Atril, but also in some other projects like the music scorewriter Denemo.

It seems that Evince libdocument has not been maintained for 13 years and is prone to the year 2038 problem.

I propose to separate the backend library libdocument from the frontend application Atril and use it as a common library for Evince, Atril, Xreader, Lector and Denemo.