Support maximum number of entries

[felixmoeller]

As disscussed on the Developer meeting there needs to be a possibility to limit the number of phone numbers for example.

There are phones that just support for example three phone numbers but in KAdressbook one might store way more numbers.

It should be specify which of these numbers should be send to the device or at least there should not be send more numbers than supported.

[dgollub]

Suggestion is that we could use the merger to store interanl "extensions" to select/mark entrise to sync.