Flag property info is awkward

#1315850: Add support for Entity API properties added flags as entity properties, which is great because it allows for integration between Flag and Facet API and Search API. However, the label for global flag properties is long and awkward:

  $property_definition = array(
    'label' => t('Whether the entity is flagged with flag @flag', array(
      '@flag' => $flag->name,
    )),
  );

This means when you have a facet on the flag property, you get a header that looks like "Whether the entity is flagged with flag flag_machine_name" with links for "true" and "false" following it. If you want to have facets for several or many flags, that's really verbose and exhausting to look at.

Furthermore, this information is already displayed in the property definition, which is what it sounds like anyway. Let's change the property label to the flag's natural label, [code]$flag->title[/code].