xdg-email: Can only send one attachment

[smcv]

The reference (shell script) implementation of xdg-email can take more than one attachment by repeating --attach, but this implementation treats each --attach as overwriting the previous one.

Fixing this will require a bit of refactoring (turning --attach into a G_OPTION_ARG_FILENAME_ARRAY, and iterating over the array to populate an array of handles) so I'd prefer to get #51 in first.