Fix EZP-23425: Pasting at the end of paragraph in Chrome put the text at the beginning

JIRA: https://jira.ez.no/browse/EZP-23425

# Description

As the title says, if you paste something at the end of a paragraph with Chrome after adding a space, the pasted text appears at the beginning of the paragraph. This is a *regression* caused by the patch for [EZP-21903](https://jira.ez.no/browse/EZP-21903) (see https://github.com/ezsystems/ezpublish-legacy/pull/834) that happens only in recent Chrome version.

The patch makes sure the text is pasted at the correct place by [using a *selection bookmark*](http://www.tinymce.com/wiki.php/API3:method.tinymce.dom.Selection.getBookmark).

# Tests

manual tests in Chrome 38