[5.x] Allow Duplication of Entry Types

[DowleyDeveloped]

With the merge of Matrix Blocks into Entry Types (Loving it btw!), it would be really handy if we could duplicate entry types themselves.

We have a few components which have the same fields (layout etc) and it would be much easier to just duplicate existing types instead of build from scratch each time.

[piotrpog]

Yeah, just add "save as new entry type", just like we can with cloning regular entries. Would be pretty useful.