ECMS-6382 | Shift Enter and Enter do the same thing in CKEditor

**Problem Analysis:**
* `shiftEnterMode` option is always set to the same value as the `enterMode`.

**Fix Description:** 
* Add a new instance variable in the `org.exoplatform.ecm.webui.form.field.UIFormRichtextField` to hold the `shiftEnterMode` retrieved from the dialog template options.