optional ability to remove link by leaving URL empty

[Paul Moers]

Allow developers to configure whether they want their users to be able to remove a link by leaving the URL empty in the link dialog.

For example a setting 'FCKConfig.LinkDlgRemoveLinkByEmptyURL' could be provided. The contextmenu should also listed to the setting and not show the unlink item when the setting is true; or perhaps another setting should be created to be able to specify this separately; or, yet another option, link and unlink could become separate items that need to be specified individually in FCKConfig.ContextMenu.

Removing the link can be easily done by not alerting an errormessage when the URL is empty but using 'oEditor.FCKUnlinkCommand.prototype.Execute();'. This needs to be done for the lines 493, 506 and 522 in 'editor/dialog/fck_link/fck_link.js'.