Feature: duplicate layer

[juliandescottes]

We should allow users to completely duplicate the content of a layer.

Given that the layer toolbar is really crowded right now, we need first to rework this UI before this can be implemented in the UI.

[juliandescottes]

Technically speaking, duplicating the currently selected layer is very easy to do with our utils, so let's first get a util ready in LayerUtils, get some unit tests on it and expose it via a bookmarklet.

[juliandescottes]

As suggested by a user via email, we could also expose it via shift + click on the "+" button of the layers toolbar!

[juliandescottes]

Released with v0.11.0