.h files: Add missing _get_type functions
transfers: Add _ERROR state

When setting the requested state of a transfer fails, the state is set
to _ERROR now, instead of _KILLED_BY_US. Outgoing file transfers no
longer just disappear from the chat widget on error, so the user can see
that an error occurred.
transfers: Save-as icon for accepting transfers

Use the document-save-as-symbolic icon instead of the media start icon
for incoming file transfers. Also use the suggested-action style class
to guide the user a bit how to start the transfer.
transfers: Change icon for closing transfers

Use edit-delete-symbolic instead of window-close-symbolic, it fits the
context better.
transfers: Hide progress bar in case of error/kill

It's not needed then, and makes the wrong impression that the transfer
might still be running.