MDL-37459 Admin form defaults - freeze locked elements and add icon.
MDL-37459 Admin form defaults - Use setConstant when value is locked.
MDL-37459 Admin form defaults - Refactored advanced and locked flags into an admin_setting_checkbox class

Makes the API a little cleaner and removes some duplication.
MDL-37459 Admin form defaults - Moved the flags to an array of flags.

Also added an option for an 'enabled' flag (I need it for duration defaulting).
MDL-37459 Admin form defaults - Fix up after refactoring checkbox => flag
MDL-37459 Admin form defaults - Add missing language string
MDL-37459 Admin form defaults - Allow a duration field to be applied as the default for a date time field

This adds to duration to midnight on the current day to set the default for the form. You can enable an
"enabled" flag on the duration field to set the default for the enabled field on the date time field.
MDL-37459 Admin form defaults - Un-deprecate the _with_advanced and _with_lock custom settings.

They are really thin wrapper classes now and are still convenient because they can be used in a single line.
Deprecating them and changing all the existing uses in Moodle would be a large patch for no benefit.
MDL-37459 Admin form defaults - Provide example in config-dist of forcing flags on a setting.
MDL-37459 Admin form defaults - Remove unused variable.
MDL-37459 Admin setting defaults - More API cleanup
MDL-37459 Admin form defaults - Remove unused consts