Allow to choose the VTE cursor shape

A user on IRC asked for the possibility to choose the VTE cursor shape, so here's a implementation.

I'm not 100% sure we want to, or how to support that since the feature is only available in VTE 0.20 and we currently depend on some version earlier than 0.17.  What I did here is to make the setting insensitive if not supported, but maybe we would rather hide it completely, or simply not support this feature -- or only support it as a hidden preference?

So I'm making a PR for you to give your opinion on this.