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!
Wouldn't a simple
````
if (is_dir(JPATH_COMPONENT . '/models/fields'))
{
	JForm::addFieldPath(JPATH_COMPONENT . '/models/fields');
}
````
do the same?