Make it return bool since the TriState was really only used signify that
the property was unsupported but there is already a separate way to
check if it's supported. More importantly there is no different set of
actions available to a user if they're in the Unknown or False state.
Because of the change to bool, we also rename the property to have an
'is'-prefix.
Change-Id: Iaaaad5ac31e663c36e00223bf5b0e719f412fc69
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>
14b74af060