Tofu in xi:include fixups, block/warning against functional xi:fallback

[alfsb]

Add tofu (U+FFFD � REPLACEMENT CHARACTER) in empty elements to avoid trip PhD into generating invalid HTML, also block (on doc-en) and warning (in translations) in the presence of reachable functional <xi:fallback>s, that cause manual parts to go silently missing.

Comments and reviews are welcome. Plan to merge this after manual split.

[alfsb]

The tofu thing is a workaround to the long standing php/phd#247 and php/phd#181 .

[jordikroon]

Please use � instead. It trips me over a little, and it's a special unicode character. Just like how & is preferred usually.

This character is also not consistently rendered across browsers and/or operating systems, but for this purpose that's not a problem.

[alfsb]

One alternative over tofu is to repeat the $alert in each inline element. Even more so now that it is "reduced", after I pushed the failed xpointer body into XML comment, that probably will not render.

That said, I prefer a single character that is not consistently rendered across browsers, than having random pages being consistently rendered wrong across browsers.

But the comment thing worries me. It will take a long time until I borrow a machine to test the PhD side, to see if this change in fact fixes both issues, and does not cause one immediate breakage.