Implement a simpler way for loading external plugin JavaScript files

[martinkou]

External plugin files are often not named plugin.js, so that CKEDITOR.plugins.addExternal() don't work for them.

The current solution is that the web app developer using CKEditor would define a separate function called CKEDITOR_GETURL() that will scan for patterns of his files and output the correct URL.

While the approach works, it is still too complicated and error-prone for many web app developers. A simpler way is needed to enable external plugins with arbitrary file names to be loaded.

[fredck]

With the proposed patch, addExternal accepts a third optional parameter for the file name. If not provided, the default name is used.

[garry.yao]

I've added test case along with the patch.

[fredck]

Fixed with [3245].