split bp-core into core and common packages

[DJPaul]

In bp-core, we have a mixture of common functions (templating, email, etc) mixed with the core architecture (our action/filter registration, upgrade functions, etc). bbPress' organisation is slightly more advanced, and has a neat split between the two.

I'd like to split bp-core into bp-common and bp-core.

[boonebgorges]

Is this mainly for code organization, or does it affect dependencies? Would there be cases in which core was loaded but not common, or vice versa? (Asking mainly for clarification - seems fine to do either way.)

Be sure to use svn cp for this so we don't lose history :-D

[DJPaul]

Organisation. Probably would have to whitelist "common" in the same places we whitelist "core' (i.e. autoloader). Both would always be loaded. Common would not show up in the Component list, for example.

[DJPaul]

This ended up being non-trivial because I couldn't decide whether to make "common" its own Component, or have it just be an empty folder, and if so, how to load it all, etc.

I guess we'll look at this in the future if there's more enthusiasm.