QtPropertyBrowser: Allow controlling whether to show text for bool values

This was a rather involved patch, but it avoids the need for using
private API or duplicating QtBoolEdit (which is in a private header).

Apart from adding support for this QtBoolEdit option to the
QtCheckBoxFactory and the QtBoolPropertyManager, the setting can also
be controlled via the 'textVisible' attribute associated with
QtVariantProperty instances.

Change-Id: Ia0c694ca852449c74cbe25429df69413211b45a5
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>