Generate documentation for internal functions

Thunar only generates GTK-doc for extension library (namely thunarx) and not thunar itself. But a documentation for internal function might be useful for several reasons:

1. Document readability

Thunar already has gtk annotations (a formatted comment on top of the function definition) for most function, but it is hard to read with that format. It may be parsed by gtk-doc generator and can be read easily.

2. Can encourage writing documentation

If a document is generated with gtk annotations, not writing a gtk annotation for a new function would seem problematic. Also, an MR only for gtk annotation would make much more sense with a presense of a document.

3. Ease a code transfer between Thunar and xfce libraries

Some parts of Thunar would make more sense when it is in libxfce4ui or libxfce4util. These two libraries generate gtk-doc, so it would be easier to move functions if they are already documented before transfer.