NetworkRequest: add a SetTimeout method.
add libshared to the development packages.

* We install the headers for many things that are currently in libshared
(BColumnListView, BCalendarView, etc). So it makes sense to also provide
the lib in an "use at your own risk" way. Only the static library is
included, so apps linking against it should continue running on newer
Haiku versions even if the content of the lib changes.
* 3rd party application developers can now make use of those
experimental features without having to copypaste and fork the sources.
UrlRequest: add empty SetTimeout for ease of use.

* Requests that support this can implement it, NetworkRequest does
* Build fix: no need to return status_t.
style fix.