Added support for tcell.Screen.SetCursorStyle

[Kavantix]

This draft adds very basic support for the new SetCursorStyle feature of tcell (not yet released as far as I can see).
We probably want to be more declarative about this though.

@mjarkk @dankox any suggestions on how to make this nicer?

Perhaps it makes sense to add a CursorStyle field to View that people can set and we then update the cursor style depending on which view is the current view?

[dankox]

This looks good to me. Nice feature.

Sorry for such late response on this one.

[dankox]

Actually this needs to be updated to 2.5.

[dankox]

Actually I didn't realize that the version doesn't match, so it needs to be updated.

Other than that, I like the design. There is not much to do to be honest. You set the style of cursor and that's it :) At least that's how I see it. The function makes sense.

[dankox]

Perhaps it makes sense to add a CursorStyle field to View that people can set and we then update the cursor style depending on which view is the current view?

I like the function more, because that way you can change the style of cursor on fly and reflect specific ways of editing for example. If it would be set as a field it would feel a bit "permanent".

However, you have a point, that maybe it could be also per View. Maybe even keep both. Global one set on gocui and View one which would make the global one changed to view when a view is active (current).
But this could be added even later. This PR or at least this current implementation definitely makes sense to me. The view implementation can be a "cherry on top". :)