Dispatch unsupported URIs to native apps on FreeDesktop.Org-compliant desktops

Improve error reporting & it's localization

Add documentation, minor exported API adjustments.

Add Accept-Language support on UNIX systems.

Add Accept header support, remove cookies support

Extract URL resolver from Rhapsode

Each URI scheme can be turned on and off at build-time,
which has the side-effect of clearly labelling the code.