Add a "fckActiveId" variable for the active editor

I'm working on a module that provides an easy way to insert uploads into the body or other textareas when editing a node. Handling textareas and tinyMCE are pretty easy, since I can keep track of textfields using onFocus and tinyMCE provides a handy tinyMCE.activeEditor variable. However FCKeditor doesn't provide such a variable, so there's no way to know which editor the user is currently editing.

This patch adds an OnFocus handler to the FCKeditor instance so that the active editor can be found with ease, using FCKeditorAPI.Instances[fckActiveId].InsertHtml(content);.