Rename RelatesTo for reply messages

Qt < 5.9 moc chokes on that

This at least partially addresses #3.  I have a feeling that
additional updates will be needed at some point, but this
fixes the issue where mtxclient would throw an exception for
unrecognized event id formats, which caused nheko to crash.

That's what the README suggests and - more importantly - the cmake
config file works with.
Also use angle brackets instead of quotation marks in two cases.