fix mpris track change

\x00 chars are not allowed in C (end of string) so in GTK. But we can get from gpg encrypted messages.

Readd part of [8904] removed in [8938]. Then mix them. So we fix #3495/#3506 and problem when connecting
with an already used nick.
Fixes problem when we change nick with dialog to an already used one and then cancel. Fixes #3506.

1/ connect, get form and disconnect
2/ re-connect, get form, check it and send filled form

re-create gpg instance in connection.py at each connection son use_gpg_agent current value of use_gpg_agent option is used
warn user when he configured gajim to use gpg-agent but it returns a wrong passphrase. see #3592

remove useless config option usegpg