[feature request] Add three-valued button

[sergeyprokhorenko]

This new feature should be used instead of a checkbox, for example, to apply or prohibit the specified search criterion from an automatically generated list of criteria.

There are only homemade solutions now that are similar to my proposal, for example, in Nigma search engine:

The three-valued button represents a control that implements the three-valued logic.

The true state is equivalent to the checked state of the checkbox.
The unknown state is equivalent to the unchecked state of the checkbox.
The false state has no equivalent for the checkbox. This state means prohibition of the specified search criterion.
The all state is equivalent to the undefined state of the checkbox.

These states must be looped (but it is preferable to use a dropdown list/menu of states):

• unknown -> true -> false -> unknown
• all -> true -> false -> unknown -> all (for group button)

[sergeyprokhorenko]