Consider editor policy only if format does not offer policy

[reckart]

Describe the refactoring action
Currently, the editor and the format can both provide content policies and they stack. However, this means that for an element to get through, it must be whitelisted for the editor as well as for the format. This in turn means that supporting any new format (with a given policy) will also require a new editor (with a compatible policy) or an editor without any policy. But it should be possible to build editors and formats more or less independently.

Thus, it seems sensible to allow preferring either the editor or the format policy and only use the preferred one if it is available. The proposal: if the format already brings a policy, then the editor policy could be ignored.

Expected benefit
This allows developing editors with policies that can operate in conjunction with the generic XML support. It also allows developing new formats with policies that can operate in conjunction with reasonably generic editors.