Use exposed label for boolean filters when labels are disabled

Problem/Motivation

When filter labels are disabled, boolean filters only display true/false or yes/no in the summary.

It makes them mostly unusable.

Steps to reproduce

Create a boolean exposed filter and disable labels in the summary.

Proposed resolution

Make the VEFS module compatible with the Better Exposed Filters "Single On/Off" (bef_single) widget, allowing to display the filter exposed label when the filter value is true.