v0.29.6: bump version to 0.29.6

update to libssh 0.9.4, see https://www.libssh.org/2020/04/09/libssh-0-9-4-and-libssh-0-8-9-security-release/

Still shown as 0.9.3 in about dialog, due to https://bugs.libssh.org/T226