This introduce a `_getProperty` method that's not really part of public
API, but could be called to get a property without security check.
Because some classes were already using internally a _getProperty method
for a different meaning, these methods have been renamed.

This also re-enable parts of test_PropertySheetSecurityOnAccessors that
are supposed to work (security is checked on accessors) and split
what's currently not supposed to work (security on accessors does not
work in the corner case of using the default read permission, 'Access
contents information' as a write permission) to another expected failure
test.