EZP-23501 implement keyword field editing

## Description

The goal here is to be able to fill the keyword fields in the edit part of the platformUI. Currently the separator is the coma ( "," ). The potential spaces around coma that the user could have put in the input text box are automatically deleted before being inserted in the field value.

##Screenshot

![image](https://cloud.githubusercontent.com/assets/5558766/4903735/efe6303a-6442-11e4-8da3-519213365bf2.png)

When tag is requiered and input text is empty : 

![image](https://cloud.githubusercontent.com/assets/5558766/4903636/fba7d280-6441-11e4-9dd2-40bcb481a360.png)

## Link

https://jira.ez.no/browse/EZP-23501
it seems like PhpStorm does not like this code ;-)