This change ensures that the correct default value of 0 is used and
that values greater than 2147483647 will fall back to the default value.
It also splits the display size concept into a separate method, as
this isn't supposed to be used when getting the IDL property.
Currently the `<select>` dropdown IPC uses the option value attr to
find which option is selected. This won't work when options don't
have values or when multiple options have the same value. Also the
`SelectItem` contained so weird recursive structures that are
impossible to create with HTML. So I refactored `SelectItem` as a
variant, and gave the options a unique id. The id is send back to
`HTMLSelectElement` so it can find out exactly which option element
is selected.
