Combine Privatemsg and privatemsg_filter into one module

Ok here goes the rationale:

The original idea was that there would be a core module and other modules to extend the core module.

However, we are using the privatemsg_filter module to provide an inbox for those users that want one. This causes a problem:

When enabling the inbox - or rebuilding it - we need to know which messages are supposed to be in the inbox. we do this by analysing the data in the index folder.

Since the filter module can be turned off and on at any time, we cannot then fix {pm_index} into some simpler form that may not contain a way to find this data and in our case, we will always have to have duplicate records in the index so that the filter can recreate the inbox.

On the other hand, having filter in the core module does not allow for alternative implementations.

Discuss.