String editor mode

The ability to click on any t()'d string and edit it and save it.

If you go the X-Cart demo site, log in, and then click on Webmaster mode you can see such an editor in action.

Two versions are needed. One is a non-JS solution, realized with a popup which contains a normal form. Reuse the existing string edit form.

The JS version should not open new windows, instead the same edit form should appear inline (in a nice DIV, likely) and submit back via AJAX (we have a lean AJAX API).

It's possible that this task needs core patching. At least it's (very) hard to do without.