core/ui: add '--keysize' option to handle non-default key sizes (bgo #706775)

If openvpn server is configured with a non-default key size, clients have to use
the same size too. We add an option to use custom key size in GUI and handle that
in the service and properties code including import/export too.
However, note that using key sizes different from cipher-specific defaults is
not recommended. List available ciphers and default key sizes by
'openvpn --show-ciphers'.

man openvpn: search --keysize

https://bugzilla.gnome.org/show_bug.cgi?id=706775
properties: simplify code dealing with check/spin button changes, saving code