Tinymce plugin: update to version 3.5.8 of Tinymce editor

Second try: PR for branch Elgg:1.8.

This PR updates the tinymce editor included in the bundled tinymce plugin from version 3.4.7 to version 3.5.8.

Between version 3.4.7 and version 3.5.8 quite a lot of bugs have been fixed in the tinymce editor. Some of the fixed issues are browser specific, others are more general. I think it's a good idea to keep the bundled tinymce plugin up with the latest release available of the tinymce editor, especially since some of the issues that were fixed had been really annoying (and still are for example on the Elgg community website!).

No other changes apart from updating the tinymce folder in the vendor directory are necessary. The new version works well (tested already for quite some time within my extended_tinymce plugin).