[FIX] web: datetime widget didn't detect all changes

The datetime widget use the "keypress" event to detect if it has been
changed.  Unfortunately, some keys, such as backspace, do not fire a
keypress event.  This commit listen instead on the 'change' event, which
should fire when the input has changed.

addons/web/static/src/js/widgets/date_picker.js

@@ -14,7 +14,7 @@ var DateTimeWidget = Widget.extend({
     events: {
         'dp.change .oe_datepicker_main': 'change_datetime',
         'dp.show .oe_datepicker_main': 'set_datetime_default',
-        'keypress .oe_datepicker_master': 'change_datetime',
+        'change .oe_datepicker_master': 'change_datetime',
     },
     init: function(parent) {
         this._super(parent);