change plugins keyfile path to XDG Base Directory spec

[f321x]

changes the plugins keyfile to either XDG_CONFIG_HOME/electrum/plugins_key, or if empty to HOME/.config/electrum/plugins_key or if HOME is not set either (which should not happen in practice), to /etc/electrum/plugins_key.

Also adds a bring_to_front() call after installing external plugin so the PluginsDialog is raised back in front.

see: https://specifications.freedesktop.org/basedir-spec/latest/ fixes #9922

[ecdsa]

I am not expert enough to tell whether this is safe. It seems to me that the permissions/owner of a file located in a user-owned directory might be less restricted than in a root owned directory.

I think it is safer to postpone this for now. There might be other ways to address #9922