Default to "yyyy-MM-dd" for date format, and "H:mm" for time_format.
Switched to config API for setting/getting values.
Avoid using "MMM" in particular, since it causes
inconsistencies in month names between jQuery UI and Zend_Locale_Format.
Fixes https://github.com/silverstripe/silverstripe-cms/issues/544
It doesn't make a lot of sense to limit the listing
to assets/Uploads/, which is the default set through FileField->folderName.
Showing all files regardless of folder makes them easier
to find, users can still opt-in to filtering by folder
through the TreeDropdownField.
DataObject::validate() is currently set to protected, but this means
you can't call validate() from outside the context of itself unless
you overload the method to use a public visibility and then call
parent::validate()
As it would turn out, most classes that overload this method already
set the visibility to public, so it would make sense the parent matches
that as well.
