- CGI script and index.html are moved from gluon-web to
  gluon-config-mode-core, the script is renamed to 'config'
- gluon-web and gluon-web-model base views and i18n files are symlinked
  into the new path
- gluon-web-theme is renamed to gluon-config-mode-theme and installs
  directly into the new path
- all gluon-web-* models, controllers and views are moved into the new
  path

Also bring back gluon-web-theme's i18n strings.

exec() is moved to gluon.util.

By emitting Lua code to call translate() and pcdata(), we are more
flexible than when doing this internally in the parser. The performance
penalty should be negligible.

We need a bit less escaping this way.

Also fix all warnings.

Open with O_CLOEXEC, use fstat() instead of stat().

Our strings can be looked up verbatim.

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.