Fixes: cfe1bba8 "gluon-web: fix radio button view of ListValues"

Fixes validation of ListValues.

Fixes: ec532b95 "gluon-web: extend ListValue with optional and unset
values"

If a value is unset or optional, an empty choice is added to the selection.
This empty choice will be marked as invalid if the value is not optional.

This is properly supported for the 'select' widget only for now, and not
for 'radio'.

Pretty much everything about this was broken:
* Fix dependency tracking
* Fix vertical orientation
* Fix paddings
* Add theming

Doing so caused broken validations, as different validators were affecting
each other.

_translate() will return nil when no match is found.

The gluon-web package is basically a stripped-down and refactored version
of the LuCI base.