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
a bit weird to have an array here as `newValue` is mainly use to set the value in the input. It would be better to have a string here like `Keyword, key, word` for instance. (same is valid for the other test below)