Add fullscreen API

[MortimerGoro]

• Receive DOM fullscreen state via wpe_view_backend_set_fullscreen
• Dispatch exit/enter fullscreem from libwpe to the client (e.g. WPEView in Webkit)

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Corresponding WPE WebKit patch: https://bugs.webkit.org/show_bug.cgi?id=227951

[aperezdc]

Functionality-wise I think the patch looks fine, though it was a bit hard to follow what's going on and how the API is to be used without looking at the accompanying patch for WebKit and the PR for Cog. Would it be possible to add some reference documentation?

[aperezdc]

This is starting to look ready for merging, thanks for working on it! 💪🏼

I have left a few comments with suggestions, and it would be great if we could have @zdobersek and/or @carlosgcampos check this and rubber-stamp that they are okay with the new API.

[aperezdc]

This looks almost ready, thanks! It will only need a couple of touch ups in the documentation comments.

In the meantime, I will try to get someone else to do a review as well, I would prefer to have one more person agree on new API before merging.

As a side note, I noticed a bug in HotDoc while trying to build the documentation added by this MR locally, and I have reported it: hotdoc/hotdoc#230 🙃

[aperezdc]

This looks mergeable to me. Waiting to have a second opinion on the new API, hopefully @zdobersek and/or @carlosgcampos can take a look soon 👍🏼

[aperezdc]

Thanks @carlosgcampos for commenting, and @MortimerGoro for updating the PR once again! I think we can consider this now approved, right?

[aperezdc]

Let's merge this, I would like to make a development release including this feature, and it looks like @carlosgcampos should be happy now with the function name changes applied. If there are any other tweaks on top of this we may want before 1.12, we can use follow-up PRs.