Fix actual & potential crashes.

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