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.

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

Fix defaults for XDG environment variables.

Draft logic for looking apps to dispatch MIMEtypes & URI schemes to on XDG platforms

Reverse locale reformatting for XDG config localized lookup.

Implement lookups of ini-file keys.

Draft parser for FreeDesktop.Org config files.

This'll be used for dispatching URLs to be opened in external apps.