Disabled rows in tableselect elements

Problem/Motivation

There's no handy way to mark some rows as disabled for selection in a tableselect element.

Proposed resolution

For tableselect elements with #multiple === TRUE, allow to pass an optional #disabled property in option.. When an option has #disabled === TRUE, the corresponding checkbox will be disabled. The new option doesn't have any effect for radio buttons (when #multiple === FALSE).

Remaining tasks

None.

User interface changes

None.

API changes

A tableselect element can have disabled options.

Data model changes

None.

Release notes snippet