Allow custom fields in plugins

### Simple addition to allow plugins to use custom fileds in a sub-directory named ```fields```.

#### There shouldn't be any B/C breaks here.

##### How to test
apply this patch
apply also patch #4240
open /plugins/editors/tinymce/tinymce.xml and remove:
```addfieldpath="/plugins/editors/tinymce/fields”```
from the line:
```<fieldset name="basic" addfieldpath="/plugins/editors/tinymce/fields”>```
Go to admin and observe that the fields site skins and administrator skins are still functional.
Also try 3PD plugins don't misbehave!