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 alternate 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 one more ```if``` statement for every template, module, plugin in the pile of execution code, which for all is a check if directory exists. (in theory this should have unnoticeable performance degrade, unless the server has really slow hard disk)

#### 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 #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:
To test the component functionality open administrator/com_banners/models/forms/banner.xml and remove addfieldpath="/administrator/components/com_banners/models/fields” from <fieldset name="details”>. Try to edit a banner and check on the banner details tab the functionality of client and impressions. Those are the custom fields here.

Also try that 3PD plugins, modules and templates don't misbehave!
@Bakual Unfortunately the ```protected static function addPath($entity, $new = null)``` from joomla/form/helper.php doesn’t do that check. So if we remove that check then it would be granted and added  to every plugin, template, module, component and I am not sure that we want to do that. Shall we remove them?