Introduce predefined path in plugins, modules, templates for custom fields in the manifest file

### Simple addition to allow components, plugins, modules and templates to use custom fileds in a sub-directory named ```fields```.

This is an alternative way to use custom fields in these areas besides the classic hardcoding ```addfieldpath=“wherever"```. Should make things a bit easier for devs without introducing complicated and slow procedures (only one directory check is actually used).

#### Pros
Predefined workflow
Easier implementation
No need for hardcoded paths in xml
Unified method that works along the existing hand coded ```addfieldpath=“wherever"```

#### Against
Adds one more directory with predefined name
Adds some more calls for every template, module, plugin in the pile of execution code.

#### There shouldn't be any B/C breaks here.

##### How to test
For plugins:
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.

For templates check the instructions at #4295

For modules repeat the instructions of plugins but for any module you choose. For a custom field code just use the one from the tinymce and it should work (you only need to see it rendering the functionality is not essential here). So for mod_breadcrumbs:
open mod_breadcrumbs.xml and paste 
```				<field name="skin_admin" type="skins"
				       description="PLG_TINY_FIELD_SKIN_ADMIN_DESC"
				       label="PLG_TINY_FIELD_SKIN_ADMIN_LABEL"
						>
				</field>```
under the fieldset=“basic”
copy the folder ```fields``` from the plugin tinimce [the plugin test abobe]
check in the admin module breadcrumps for the new option

For components:
This is already the standard for components. Custom fields expected to be found at ```com_name/models/fields```

Also try that 3PD plugins, modules and templates don't misbehave!
That doesn't work that easy. JPATH_BASE will be pointing to admin when the module is edited in backend and to site when it is edited in frontend. At the same time we have both backend and frontend modules.
So what you need here is load the folder depending on the client_id:
````
$baseFolder = ($clientId) ? JPATH_ADMINISTRATOR : JPATH_SITE;
JForm::addFieldPath($baseFolder . '/modules' . '/' . $module . '/fields');
````