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