Fix timezone handling in Date widget

The date displayed by the Date widget when a default time is set is not correct: the shift which corresponds to the user timezone is not understandable. Here is an example:

Pre-requisites:

• Set the Timezone on your computer to PST

   export TZ='America/Phoenix'

  You will use the console with this environment variable to start Nuxeo

Steps to reproduce:

1. Define a Date field
2. In the corresponding JSF layout, use a datetime widget configured as below

         <widget name="mydate" type="datetime">
           <labels>
             <label mode="any">Mydate</label>
           </labels>
           <translated>false</translated>
           <fields>
             <field>schema:mydate</field>
           </fields>
           <properties widgetMode="edit">
             <property name="format">yyyy-MM-dd HH:mm</property>
             <property name="timeZone">UTC</property>
             <property name="defaultTime">05:00</property>
           </properties>
         </widget>
   

• Load this configuration
• Create a document which contains this Date field
• Click on the Calendar and select a date, for example August 15th
• Observe that the date displayed in the field is

  2018-08-15 22:00

=> the hour corresponds to the defaultTime value minus the PST Timezone (7h) BUT the day is the same, whereas August 15th 5:00 AM should correspond to August 14th 10:00PM

Expected result: the date displayed should correspond to the selection