ACF should clean up empty elements

[Piotrek Koszuliński]

TC:

1. Create editor without image button (e.g. use one of editors in filter sample).
2. Switch to source.
3. Set: <p><em><img src="" alt=""></em></p>.
4. Switch to wysiwyg mode.
5. Check with dev tools that editable contains <p><em></em></p>.

There are few cases:

• <b><img></b>: after filtering out <img> entire <b> should be removed, because it's in DTD.$removeEmpty.
• <a href=""><img></a> is a different case because it's similar to <a name=""><img></a> and <a name=""></a> shouldn't be removed. So it's an exception, because 'a' isn't included in $removeEmpty. However, why doesn't data processor fix this case? It fixes the case I described in the next point, but why not this? Especially, that I see a filter's rule for htmlFilter that fixes this on output. I think that identical rule should exist for toHtml(). And perhaps previous point should also be handled by data processor...
• <p><img></p>}: after filtering out <img> filter will left empty <p></p> which then will be correctly handled by data processor, so in this case we don't have to act.

Important note: ​http://dev.ckeditor.com/ticket/9992#comment:7

[Piotrek Koszuliński]

Another uncool case - when table is allowed (e.g. by style), but tr,td are not, then table is incorrectly stripped out.

[Piotrek Koszuliński]

Pushed ​t/10073 on dev and tests.

[Piotrek Koszuliński]

Merged to major with ​git:3ba4d62 on dev and 708b379 on tests.