Integrate AppStream into normal fetch routines, version 1.3.0.0!

Draft AppStream loading code.

Fix app dispatch logic.

Previously failed to build due to a file not listed in the build system.
Previously crashed when trying to read a non-existant mimeapps file.

Report encountered exceptions in the response body.

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.

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.